This article provides an introduction to RS-422 and RS-485 interfaces and explains why you might want to use them in your projects.
Related information
- Why and how to use differential signaling
- Interrupt-friendly UART double buffering technology
Most of us are familiar with RS-232, a reliable but inconvenient standard that is forever associated with our memories of the increasingly obsolete serial port on the computer. You may be less familiar with RS-422 and RS-485, which are really (as the name suggests) related to RS-232.
However, do not make the mistake of assuming that these newer standards share the characteristics that make RS-232 so incompatible with modern electronic systems. RS-422 and RS-485 are major improvements to the RS-232 theme; both could be good choices for your next digital communication channel.
Firstly, RS-422 or RS-485
These two standards are usually grouped together because they have so much in common. But they are, of course, not identical, and RS-422 and RS-485 devices are not completely interchangeable. First, I'll discuss the significant differences between these two standards. Then, in the rest of the article, we can make a simplification by referring to them as "RS-422/485".
Both RS-422 and RS-485 allow you to use multiple devices on a bus (that is, you are not limited to one transmitter and one receiver). However, RS-422 can only be used for multi-subscriber tires, i.e. a differential pair can have multiple receivers but only one transmitter.
The maximum number of receivers on a two-wire RS-422 bus is 10 (well, sort of... see the discussion of "unit loads" below).
On the other hand, with RS-485 you can have real multipoint a system where "dot" instead of "subscriber" means that one differential pair can support multiple transmitters as well as multiple receivers.
RS-485 also increases bus capacity to 32 devices.
(Actually, it's not that simple - the standard specifies a maximum of 32 "unit loads", but you can connect many more than 32 devices using RS-485 chips, which represent only a small fraction of the unit load on the bus. It's a bit complicated, and Honestly, this is the point where I start to lose interest... But if you are more persistent than me, you can read the details.)
The fully equipped RS-485 bus provides a high-performance interface. In addition to the benefits discussed later in this article, you can have multiple transceivers that use the same two wires, and any device on the bus can send data to any other device on the bus.
Another important point is that RS-485 is an important extension of RS-422. In other words, RS-485 adds and improves functionality, but does not conflict with anything in the RS-422 standard. Thus, an RS-485 device can be used on an RS-422 network, but RS-422 devices are not necessarily compatible with an existing RS-485 network.
Basics
RS-422/485 is a four- or two-wire, full- or half-duplex, differential, medium-speed serial interface that supports multi-drop (RS-422) or multi-drop (RS-485) bus architectures. Here are some comments on these characteristics:
I like it
Characteristics of RS-422/485 - long cable lengths, noise resistance, etc. - make it an excellent choice for industrial applications. However, part of my goal in this article is to demonstrate that RS-422/485 is a good choice for many electronic and electromechanical systems, even if you don't need all the functionality it offers. My favorable view of RS-422/485 is based primarily on three considerations: ease of design, excellent support in chip data sheets and application notes, and noise immunity.
Be more simple
Despite years of experience with various serial communication protocols, UART is still my favorite. It's simple and reliable, requires minimal interconnection, and I wouldn't be surprised to find it supported by every microcontroller on the market. It may be a little primitive, but you can always write firmware to implement any flow control, device identification, or error checking in your specific application.
Anyway, my point is that I enjoy using UART whenever I can, and RS-422/485 is a great physical layer for UART communication.
Expert support
Incorporating RS-422/485 into your project is easy: almost all you need is a converter/transceiver chip, and there are many to choose from. These devices convert standard logic signals into differential RS-422/485 signals, and also handle the rest of the pesky details required to achieve RS-422/485 compliance. And if you're unsure about exactly how to design your particular communications bus, you'll find plenty of guidance in application notes and datasheets.
RS-485 and RS-422 interfaces are described in standards ANSI EIA/TIA-485-A and EIA/TIA-422. The RS-485 interface is the most common in industrial automation. It is used by industrial Modbus networks, Profibus DP, ARCNET, BitBus, WorldFip, LON, Interbus and many non-standard networks. This is due to the fact that in all main indicators this interface is the best of all possible at the current level of technology development. Its main advantages are:
- two-way data exchange over just one twisted pair of wires;
- work with several transceivers connected to the same line, i.e. the ability to organize a network;
- long communication line length;
- quite high transfer speed.
2.3.1. Construction principles
Differential signal transmission
The RS-485 interface is based on differential transmission method signal, when the voltage corresponding to the level of a logical one or zero is not measured from ground, but is measured as the potential difference between two transmission lines: Data + and Data - (Fig. 2.1). In this case, the voltage of each line relative to ground can be arbitrary, but should not go beyond the range -7...+12 V [ - TIA ].
The signal receivers are differential, i.e. perceive only the difference between the voltages on the Data + and Data - lines. When the voltage difference is more than 200 mV, up to +12 V, it is considered that the line is set to a logical one; at a voltage less than -200 mV, up to -7 V - a logical zero. The differential voltage at the transmitter output, in accordance with the standard, must be at least 1.5 V, therefore, with a receiver response threshold of 200 mV, the interference (including the voltage drop across the ohmic resistance of the line) can have a swing of 1.3 V above the 200 mV level. Such a large margin is necessary for operation on long lines with high ohmic resistance. In fact, it is this voltage margin that determines the maximum length of the communication line (1200 m) at low transmission rates (less than 100 kbit/s).
Due to the symmetry of the lines relative to the “ground,” interference is induced in them, similar in shape and magnitude. In a receiver with a differential input, the signal is isolated by subtracting the voltages on the lines, so after subtraction the noise voltage is zero. In real conditions, when there is a slight asymmetry of lines and loads, the interference is not completely suppressed, but is significantly attenuated.
To minimize the sensitivity of the transmission line to electromagnetic interference, a twisted pair of wires is used. Currents induced in adjacent turns due to the phenomenon of electromagnetic induction, according to the “gimlet rule,” turn out to be directed towards each other and are mutually compensated. The degree of compensation is determined by the quality of the cable and the number of turns per unit length.
"Third" output state
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Rice. 2.1. Connecting three devices with an RS-485 interface using a two-wire circuit |
The second feature of the D (D - “Driver”) RS-485 interface transmitter is the ability to switch the output stages to the “third” (high-resistance) state with a signal (Driver Enable) (Fig. 2.1). To do this, both transistors of the transmitter output stage are turned off. The presence of the third state allows half-duplex exchange between any two devices connected to the line using just two wires. If in Fig. 2.1 the transmission is performed by the device, and the reception is performed by the device, then the outputs of the transmitters are transferred to a high-resistance state, i.e., in fact, only receivers are connected to the line, while the output impedance of the transmitters does not shunt the line.
The transfer of the interface transmitter to the third state is usually carried out by a signal RTS (Request To Send) COM port.
Four-wire interface
The RS-485 interface has two versions: two-wire And four-wire. Two-wire used for half duplex transfers(Fig. 2.1), when information can be transmitted in both directions, but at different times. For full duplex (duplex) transmissions use four communication lines: two transmit information in one direction, and two others transmit information in the opposite direction (Fig. 2.2).
The disadvantage of a four-wire (Fig. 2.2) circuit is the need to strictly specify the master and slave devices at the system design stage, while in a two-wire circuit any device can act as both a master and a slave. The advantage of a four-wire circuit is the ability to simultaneously transmit and receive data, which is sometimes necessary when implementing some complex exchange protocols.
Echo Reception Mode
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Rice. 2.2. Four-wire connection of devices with RS-485 interface |
If the receiver of the transmitting node is turned on during transmission, then the transmitting node receives its own signals. This mode is called "receive echo" and is usually set by a microswitch on the interface board. Echo reception sometimes used in complex transmission protocols, but more often this mode is disabled.
Grounding, galvanic isolation and lightning protection
If the RS-485 ports connected to the transmission line are located at a large distance from one another, then their ground potentials can vary greatly. In this case, to avoid breakdown of the output stages of the microcircuits transceivers(transceivers) interface should use galvanic isolation between the RS-485 port and ground. If the ground potential difference is small, in principle, a conductor can be used to equalize the potentials, but this method is not used in practice, since almost all commercial RS-485 interfaces are galvanically isolated (see, for example, the NL-232C converter or interface repeater NL-485C from RealLab!).
The interface is protected from lightning using gas-discharge and semiconductor protection devices, see section "Protection against interference".
2.3.2. Standard parameters
Recently, many RS-485 interface transceiver chips have appeared, which have broader capabilities than those established by the standard. However, to ensure compatibility of devices with each other, it is necessary to know the parameters described in the standard (see Table 2.2).
2.3.5. Eliminating Line Uncertainty
When the transmitters of all devices connected to the line are in the third (high-resistance) state, the logical state of the line and the inputs of all receivers is undefined. To eliminate this uncertainty, the non-inverting input of the receiver is connected through a resistor to the power bus, and the inverting input is connected to the ground bus. The resistor values are chosen such that the voltage between the inputs becomes greater than the response threshold of the receiver (+200 mV).
Since these resistors are connected in parallel to the transmission line, to ensure the line matches the interface, it is necessary that the equivalent resistance at the line input be equal to 120 Ohms.
For example, if the resistors used to eliminate line uncertainty are each 450 ohms, then the line termination resistor should be 130 ohms, then the equivalent circuit resistance would be 114,120 ohms. In order to find the differential line voltage in the third state of all transmitters (see Fig. 2.6), you need to take into account that another 120 Ohm resistor and up to 32 receivers with an input differential resistance of 12 kOhm are connected to the opposite end of the line in the standard configuration. Then, at the supply voltage (Fig. 2.6), the differential line voltage will be equal to +272 mV, which satisfies the requirement of the standard.
2.3.6. Through currents
In a network based on the RS-485 interface, there may be a situation where two transmitters are turned on simultaneously. If one of them is in the state of logical one, and the second is in the state of logical zero, then a large “through” current flows from the power source to the ground, limited only by the low resistance of the two open transistor switches. This current can damage the transistors in the transmitter's output stage or cause their protection circuit to trip.
This situation is possible not only due to gross errors in the software, but also if the delay between the moment of turning off one transmitter and turning on the other is incorrectly set. The slave device should not transmit data until the sending device has finished transmitting. Interface repeaters must detect the beginning and end of data transmission and, in accordance with them, switch the transmitter to the active or third state.
2.3.7. Cable selection
Depending on the transmission speed and the required cable length, you can use either a cable specially designed for the RS-485 interface, or almost any pair of wires. The cable, designed specifically for the RS-485 interface, is a twisted pair with a characteristic impedance of 120 Ohms.
For good suppression of emitted and received interference, it is important to have a large number of turns per unit cable length, as well as identical parameters of all wires.
When using non-isolated interface transceivers, in addition to the signal wires in the cable, it is necessary to provide another twisted pair to connect the grounding circuits of the connected interfaces. If there is galvanic isolation of the interfaces, this is not necessary.
Cables may or may not be shielded. Without experimentation it is very difficult to decide whether a screen is needed. However, considering that the cost of a shielded cable is not much higher, it is always better to use a cable with a shield.
At low transmission speeds and at direct current, the voltage drop across the ohmic resistance of the cable plays an important role. Thus, a standard cable for the RS-485 interface with a cross-section of 0.35 sq. mm has an ohmic resistance of 48.5 * 2 = 97 Ohms with a length of 1 km. With a terminal resistor of 120 Ohms, the cable will act as a voltage divider with a division factor of 0.55, i.e. the voltage at the cable output will be approximately 2 times less than at its input. This limits the permissible cable length for transmission speeds less than 100 kbit/s.
At higher frequencies, the permissible cable length decreases with increasing frequency (Fig. 2.7) and is limited by cable losses and the effect front tremors impulses. Losses consist of the voltage drop across the ohmic resistance of the conductors, which increases at high frequencies due to the displacement of current to the surface (skin effect) and losses in the dielectric. For example, the signal attenuation in Belden 9501PVC cable is 10 dB (3.2 times) at 20 MHz and 0.4 dB (4.7%) at 100 kHz with a cable length of 100 m.
2.3.8. Pushing the limits
The RS-485 standard allows the connection of no more than 32 receivers to one transmitter. This value is limited by the power of the transmitter output stage with a standard receiver input impedance of 12 kOhm. The number of loads (receivers) can be increased using more powerful transmitters, receivers with higher input impedance and intermediate signal repeaters (interface repeaters). All of these methods are used in practice when necessary, although they go beyond the requirements of the standard.
In some cases, you need to connect devices over a distance of more than 1200 m or connect more than 32 devices to one network. This can be done using repeaters ( repeaters , repeaters) interface. The repeater is installed between two segments of the transmission line, receives the signal of one segment, restores the edges of the pulses and transmits it using a standard transmitter to the second segment (Fig. 2.5). Such repeaters are usually bidirectional and galvanically isolated. An example is the NL-485C repeater from RealLab! . Each repeater allows you to add 31 standard devices to the line and increase the line length by 1200 m.
A common method for increasing the number of line loads is to use receivers with higher input impedance than the EIA/TIA-485 standard (12 kΩ). For example, with a receiver input impedance of 24 kOhm, 64 receivers can be connected to a standard transmitter. Transceiver chips for the RS-485 interface are already being produced with the ability to connect 64, 128 and 256 receivers in one network segment (www.analog.com/RS485). Note that increasing the number of loads by increasing the input impedance of receivers leads to a decrease in the power of the signal transmitted along the line, and, as a consequence, to a decrease in noise immunity.
2.3.9. RS-232 and RS-422 interfaces
The RS-422 interface is used much less frequently than RS-485 and, as a rule, not for creating a network, but for connecting two devices over a long distance (up to 1200 m), since the interface RS Fig. 2.9. Connection of two RS-232/RS-422 interface converter modulesDifferential
Differential
Maximum number of receivers
Maximum cable length
Maximum transfer speed
30 Mbit/s**
Common Mode Output Voltage
Line voltage under load
Load impedance
Leakage current in the "third" state
Allowable range of signals at the receiver input
Receiver sensitivity
Receiver input impedance
Note. **Transmission speed of 30 Mbit/s is provided by modern element base, but is not standard.
* EIA- Electronic Industries Association - association of the electronic industry. TIA - Telecommunications Industry Association - association of the telecommunications industry. Both organizations develop standards.
Maxim is a world leader in the production of interface microcircuits of various functional organizations.
All microcircuits have features that make it possible to reduce cost, increase the density of elements on the board by reducing the number of additional elements, and also provide a variety of protection for devices in the communication line.
In the MAXIM line of interface chips you can find:
- Transceivers for the most common industrial interfaces: RS-232, RS-485/RS-422, IrDA, CAN, LIN, LVDS, USB, HART;
- Dual-protocol devices that allow you to connect devices with different interfaces, for example RS-232 and RS-485, using a single chip;
- Multi-protocol devices supporting the following interfaces: RS-232, RS-449 RS-485 RS-530, RS-530A, V.10, V.11, V.28, V.35, V.36, and X.21;
- Microcircuits for protecting communication lines from electrostatic overvoltage, allowing for current protection of microcircuits and devices;
- Microcircuits for monitoring interface buses, allowing you to respond to short circuits in the circuit and, if necessary, connect backup power to the device being developed;
- Chips that simplify work with smart cards, as well as interface controllers that speed up the creation of USB and SCSI devices;
- I/O port expanders;
- Bilateral high-speed logic level converters for interfacing microcircuits with different power supply within the same board.
Mainly RS-485 and RS-232 interfaces are used to communicate industrial devices. Maxim's line of transceivers for these interfaces contains more than 300 different devices.
RS-485 protocol
The RS-485 protocol was jointly developed by two associations: the Electronics Industries Association (EIA) and the Telecommunications Industry Association (TIA). Previously, the EIA labeled all of its standards with the prefix "RS" ( Recommended Standard— Recommended standard). Many engineers continue to use this designation, but the EIA/TIA has officially replaced "RS" with "EIA/TIA" in order to make it easier to identify the origin of its standards.
This standard became the basis for the creation of a whole family of industrial networks, widely used in industrial automation. The main difference between RS-485 and RS-232 is the ability to combine several devices.
We list the main properties of the physical layer of the RS-485 interface:
1. Bidirectional half-duplex data transmission. The serial data stream is transmitted in only one direction at a time; transmission in the other direction requires switching the transceiver. Transceivers are usually called “drivers”.
2. Symmetrical communication channel. To receive/transmit data, two equivalent signal wires are used, which are designated by the Latin letters “A” and “B”. These wires carry sequential data exchange in both directions (alternately). When using twisted pair, a symmetrical channel significantly increases the signal's resistance to common-mode interference and well suppresses electromagnetic radiation created by the useful signal.
3. Differential data transmission method. At the output of the transceiver, the potential difference changes; when transmitting “1”, the potential difference between A and B is positive, when transmitting “0” it is negative. That is, the current between contacts A and B when transmitting “0” and “1” flows (balances) in opposite directions.
4. Multipoint. Allows multiple connections of receivers and transceivers to one communication line. But at any given time, only one transmitter must transmit data, and a large number of devices can receive data.
5. Low impedance transmitter output. The transmitter buffer amplifier has a low-impedance output, which allows the signal to be transmitted to many receivers. The standard transmitter load capacity is 32 receivers per transmitter. In addition, the current signal is used to operate the twisted pair (the higher the operating current of the twisted pair, the more it suppresses common-mode interference on the communication line).
6. Dead zone. If the differential signal level between the AB contacts does not exceed ±200 mV, then it is considered that there is no signal in the line. This increases the noise immunity of data transmission.
Differential signal transmission in RS-485-based systems provides reliable data transmission in the presence of noise, and the differential inputs of their receivers can reject significant common-mode voltages. However, additional measures must be taken to protect against the high voltage levels typically associated with electrostatic discharge (ESD).
The charged capacity of the human body allows a person to destroy an integrated circuit with a simple touch. Such contact can easily occur when laying and connecting the interface cable.
Some chips on the market do not have built-in ESD protection, which forces you to install additional protective devices on the board. Maxim's interface chips include "ESD structures" that protect the transmitter outputs and receiver inputs of RS-485 transceivers from ESD levels up to ±15 kV, and in some models up to ±30 kV.
To ensure the stated ESD protection, Maxim performs multiple tests of the positive and negative power pins in 200V increments to verify consistency of the stated levels. Devices in this class (meeting the specifications of a human body model) are marked in the product designation with the additional suffix “E”.
Also, a short circuit mode is dangerous for the output drivers of interface microcircuits, but Maxim specialists have developed a unique protection system that turns off the output drivers of the microcircuit not only when a short circuit is detected, but also when the microcircuit overheats, which ensures a long, trouble-free period of operation.
Since MAXIM microcircuits have all protection systems and level converters on one chip, the connection diagram is greatly simplified (Fig. 1). The minimum number of hanging elements allows for maximum compaction of the placement of integrated components on the board, and the minimum dimensions of communication chips (up to 2x2 mm) simplify the design of portable devices or devices operating in limited space.
Rice. 1.
Networks built on the RS-485 interface can be either full-duplex or half-duplex. Half-duplex mode is a mode in which transmission is carried out in both directions, but with a time division. At any given time, transmission occurs in only one direction. Duplex mode is a mode in which data can be transmitted at the same time as data is received. Sometimes it is also called "full duplex" mode to more clearly show the difference from half duplex.
As is known, the RS-485 standard specifies only the electrical characteristics of the communication interface and the physical layer (medium), but not the software platform. However, there are many standardized industry protocols that operate on top of the RS-485 standard. Among these protocols, the most common is PROFIBUS. It combines the technological and functional features of serial communication, which allows you to connect disparate automation devices into a single system at the level of sensors and actuators. PROFIBUS uses data exchange between master and slaves (DP and PA protocols) or between several masters (FDL and FMS protocols).
PROFIBUS DP ( Decentralized Peripheral - Distributed peripherals) is a protocol aimed at ensuring high-speed data exchange between automation systems (DP master devices) and distributed input/output devices (DP slaves).
It is characterized by minimal response time, high resistance to external electromagnetic fields and is optimized for high-speed and low-cost systems. This version of the network was designed specifically for communication between automated control systems and distributed peripherals. Electrically, the protocol is close to RS-485, which is why microcircuits that allow working using the PROFIBUS protocol can be reconfigured to work using the RS-485 interface if the user wishes.
MAX14840E and MAX14841E
MAX14840E and MAX14841E - ESD-protected transceivers designed for half-duplex RS-485 networks with data transfer rates up to 40 Mbps. These transceivers are optimized for high-speed device communications over long distances. Special systems for protection against signal asymmetry, as well as increased hysteresis of the input signal, can significantly increase immunity to interference.
The typical current consumption of microcircuits in standby mode or in operating mode (with output drivers disabled) is only 1.5 mA. Devices built on this chip can be included in an already operating network “on the fly”, without causing transient processes that worsen the shape of the currently transmitted signal.
Maxim's MAX14840E and MAX14841E are available in eight-pin SO packages and small eight-pin (3x3 mm) TDFN-EP packages, but regardless of the form factor, the chips operate in a temperature range of -40...125°C, allowing them to be used in vehicular networks.
This chip was developed to operate in a high-speed multidrop RS-485 network (Fig. 2).
Rice. 2.
The minimum number of pins of the microcircuit, as well as a high degree of internal integration, allows it to be used practically without external elements, which increases the density of the board layout and simplifies the use of the microcircuit in small-sized portable devices.
The MAX14840E and MAX14841E series chips contain an output driver protection unit that limits the output current in the event of a line short circuit, which helps keep the output drivers operational and also avoids large energy losses. This microcircuit contains an overheating protection unit that turns off the output drivers of the microcircuit when the temperature exceeds 160°C.
Main Applications:
- Engine control systems;
- Microclimate control;
- Industrial control systems;
- Various RS-485 networks.
MAX14770E
The line of microcircuits from the Maxim company includes a model MAX14770E - transceiver of PROFIBUS-DP/RS-485 interfaces. The new generation of BiCMOS process technology allows for high throughput (20 Mbps) while integrating robust ESD protection circuitry (±35 kV, HBM) into the structure. The compact TDFN package allows this chip to be used in portable devices. The microcircuit operates in an extended temperature range of -40…125°C, which guarantees reliability in difficult conditions.
The MAX14770E is pin-to-pin compatible with MAX3469, which allows it to be used for upgrading motor control systems, PROFIBUS-DP/RS-485 networks and industrial buses.
The MAX14770E has a wide industry-standard 5V ±10% supply voltage range. The microcircuits are available in a compact eight-pin TDFN package (3x3 mm), as well as an eight-pin SO package, for which the operating temperature range is -40...85°C.
Main characteristics:
- Meets the requirements of Profibus-DP supply voltage 4.5…5.5V;
- Transfer speed reaches 20Mbit/s;
- Has short circuit protection;
- Has a fail-safe receiver;
- Switches off when overheated;
- Has the ability to be hot-swappable;
- Has extended ESD protection: ±35kV (human body model); ±20kV (discharge model through an air gap); ±10kV (touch discharge model);
- Has an extended temperature range of -40…125°C for an eight-lead TDFN package (3x3mm).
Thanks to these features, microcircuits have very wide applications. In addition to devices in industrial networks and industrial equipment coding systems, these chips are actively used in motor control systems, as well as in PROFIBUS-DP networks.
MAX13181E, MAX13182E, MAX13183E, MAX13184E
Microcircuit series MAX13181E, MAX13182E, MAX13183E, MAX13184E from Maxim - RS-485 interface transceivers operating in full-duplex mode and in a selectable mode: half- and full-duplex (Fig. 3).
Rice. 3.
A special feature of these microcircuits is that they are produced in compact mDFN packages with dimensions of 2x2 mm and are intended for use in space-critical designs. Despite their size, they feature improved ±15 kV ESD protection, as well as pull-up and ground termination resistors on the DE, RE, and F inputs to reduce the number of external components.
A feature of the MAX13182E, MAX13184E chips is also a very low current in off mode, which is necessary in power-critical applications. The receiver inputs of the microcircuit create an impedance equal to 1/8 of a unit load, which makes it possible to connect up to 256 transceivers to the bus.
The MAX13181E and MAX13182E chips include slew-limiting drivers, which reduce electromagnetic interference and signal reflections caused by improper cabling. However, the use of drivers with a limited rate of rise of the output signal voltage allows data transmission at speeds of up to 250 kbit/s, although it significantly reduces the number of errors.
MAX13183E, MAX13184E, unlike previous ICs, have drivers operating at full speed, which allows data transfer rates of up to 16 Mbit/s. A special feature of these chips is the ability to select half- or full-duplex operating modes, while the MAX13182E and MAX13184E operate only in full-duplex mode. All transmitter outputs and receiver inputs have enhanced ESD protection.
All MAX13181E...MAX13184E chips are available in a 10-pin mDFN package with dimensions of 2x2 mm and in a 14-pin SO package. All of them operate in an extended temperature range of -40...85°C.
Among the features of the described microcircuits are the following:
- 10-pin mDFN package with dimensions of 2x2mm and 14-pin SO package;
- Supply voltage 5V;
- Advanced ESD protection;
- ±15kV (HBM specification - human body model);
- ±12 kV (Specification IEC 61000-4-2 - air gap discharge model);
- ±6 kV (IEC 61000-4-2 specification - touch discharge model);
- Operation mode with limited slew rate of the output signal for error-free data transmission (MAX13181E, MAX13182E);
- Low current consumption of 2.5 µA in shutdown mode;
- Impedance equal to 1/8 of a unit load, which makes it possible to connect up to 256 transceivers to the bus.
Due to their small size and low current consumption, these chips are ideal for use in portable, self-powered devices that can be used in industrial process control, instrumentation, security systems, and telecommunications equipment.
MAX13448E
MAX13448E — duplex transceivers of the RS-485 interface with protection of inputs and outputs from voltage surges of ±80 V (relative to ground). The MAX13448E operates from a 3 to 5.5 V power supply. A special feature of the IC is a protection circuit that ensures that the receiver output remains logic high in the event of a power failure or short circuit. This allows all receiver outputs connected to the bus to go high when all transceivers are turned off.
The ability to operate the IC in the presence of ±80 V voltage swings across the RS-485 interface pins eliminates the need for external protection circuitry, which typically contains resettable fuses and zener diodes.
The built-in protection circuit is successfully used in architectures such as USB and CAN, in which power and data transmission are carried out over a single cable. The MAX13448E is well suited for industrial HVAC and motor control applications.
The main feature of the MAX13448E chip is a module for limiting the slew rate of the output voltage, the use of which reduces the level of electromagnetic interference and the effect of interference on the cable, which allows for error-free data transmission at speeds of up to 500 Kbps with a power supply of 5 V and 250 Kbps with a power supply of 3. 3 V.
The MAX13448E features a hot-swappable feature that eliminates the possibility of incorrect data being transmitted at power-up or when the IC is turned on without turning off the power supply. The driver and receiver of the microcircuit have, respectively, an active high and an active low logical turn-on level, which makes it possible, when turned on together from the outside, to control the direction of transmission.
The IC receiver input impedance represents only 1/8 of the standard load, allowing up to 256 transmitters to be connected to a single bus. The outputs of all drivers are protected against electrostatic discharge up to ±8 kV (human touch - Human Body Model). The MAX13448E operates over a temperature range of -40 to 85°C and is available in 14-pin SO packages.
MAX13410E, MAX13411E, MAX13412E, MAX13413E
MAX13410E, MAX13411E, MAX13412E, MAX13413E — Half-duplex transceivers for RS-485/RS-422 interfaces, optimized for use in circuits with isolated circuits. These ICs include an integrated low-dropout voltage regulator, driver and receiver. The built-in stabilizer allows operation from an unregulated power supply up to 28 V. The function of automatic redirection of transmitted data (Maxim's AutoDirection architecture) makes it possible to reduce the number of optical elements for decoupling. Other features include electrostatic discharge protection, slew rate limiting circuitry, fault tolerance circuitry, and the ability to transfer data at maximum speed.
The built-in low-dropout voltage regulator produces a nominal 5V ±10% voltage that is used to power the internal circuits of the transceiver. The output of the built-in voltage regulator is output to VREG, which allows the user to connect external components to a stable voltage source, provided that the current consumption is less than 20 mA. The MAX13410E/MAX13411E does not have a 5V output, but its pinouts are industry standard, allowing the IC to be easily integrated into industrial systems.
With the MAX13410E, MAX13411E, MAX13412E, and MAX13413E, the IC's receiver input impedance is only 1/8 the standard load, allowing up to 256 transmitters to be connected to a single bus. The driver outputs are protected against electrostatic voltage.
A feature of the MAX13412E/MAX13413E IC is the automatic data flow redirection function. This architecture eliminates the need for DE and RE control signals.
The MAX13410E/MAX13412E utilizes slew-limiting circuitry to reduce EMI and ensure robust operation in high EMI environments at data rates up to 500Kbps. The MAX13411E/MAX13413E does not use a limiting circuit, but these chips can transmit data at speeds up to 16 Mbps.
The microcircuits operate in the temperature range of -40...85°C and are produced in 8-pin SO packages.
RS-232 interface
Despite all the positive qualities of the RS-485 interface, the RS-232 interface is still often used in industrial systems. It was designed for a simple application, as defined by its name: “Interface between terminal equipment and communications equipment using serial binary code.”
The RS-232 interface is designed to transmit information between two devices over a distance of up to 20 m. It is based on differential signal transmission, but differs in levels and polarity.
Information is transmitted over wires with signal levels different from the standard 5V, which provides greater immunity to interference. Asynchronous data transfer is carried out at a set speed when synchronized by the level of the start pulse signal.
Signals after passing through the cable are weakened and distorted. Attenuation increases with cable length. This effect is caused by the electrical capacitance of the cable. According to the standard, the maximum load capacitance is 2500 pF. The typical cable capacitance per unit length is 130 pF, so the maximum cable length is limited to approximately 17 m.
Transmitter logical levels: “0” - 5...15 V, “1” - -5...-15 V.
Receiver logic levels: “0” - above 3 V, “1” - below -3 V.
Despite the fact that the RS-232 protocol was created a long time ago, Maxim specialists are still improving the network hardware, which allows for greater reliability of industrial systems.
MAX13223E
New two-channel transceiver MAX13223E The RS-232 interface has built-in I/O protection up to ±70 V. The MAX13223E is the first surge-protected transceiver on the market that is pin-compatible with the current industry standard MAX3223E.
The new microcircuit integrates input/output protection circuits against short circuits to power rails, connection errors and overvoltage up to ±70 V, eliminating the need for external protective circuits. This protection is especially critical for applications in which power and data are transmitted over the same wire, because prevents circuit failure due to connection errors and short circuits to interface pins when the cable is damaged.
Maxim's patented AutoShutdown circuit allows the current consumption in off mode to be reduced to 1 µA. The MAX13223E automatically enters low power mode when the RS-232 link cable is disconnected or when there is no data at the receiver input. A patented efficient supply voltage pumping circuit and a low voltage drop in the transmission path ensures operation of the microcircuit from a unipolar voltage source with a nominal value of 3...5 V.
The MAX13223E, housed in a TSSOP-20 package, operates from 3V to 5.5V, providing EIA/TIA-232 and V.28/V.24 interfaces with auto-shutdown and enhanced ESD protection. The temperature range of the microcircuit is -40…85°C.
The MAX13223E is designed for use in automotive applications, communications, base stations, utility metering systems, industrial equipment, point-of-sale terminals and telecommunications equipment.
A typical connection diagram (Fig. 4) contains a minimum of hanging elements, which makes it possible to simplify the layout of the board as much as possible, as well as to maximize the compaction of the arrangement of elements on the board.
Rice. 4.
MAX13234E, MAX13235E, MAX13236E, MAX13237E
RS-232 transceivers MAX13234E, MAX13235E, MAX13236E, MAX13237E designed to replace existing transceivers of the MAX3224E...MAX3227E family and provide high data transfer rates (up to 3 Mbit/s). Built-in voltage regulators allow operation of logic levels at low supply voltages, and through the use of AutoShutdown Plus circuitry, current consumption has been reduced to less than 1 µA. The ESD circuit provides a high level of static discharge protection.
The MAX13234E...MAX13237E chips provide the ability to operate at high data rates by eliminating the need for external logic level conversion. The MAX13234E and MAX13235E chips include two receivers and two transmitters. The MAX13236E and MAX13237E include one receiver and one transmitter in a compact TQFN package. The MAX13235E and MAX13237E provide data rates up to 3 Mbps, while the MAX13234E and MAX13236E support 250 kbps operation. All devices operate in an extended temperature range of -40...85°C from a power source rated 3...5.5 V.
These chips were designed primarily for use in communications systems, but are also ideal for portable electronic devices and industrial equipment.
HART protocol
If the interfaces described above used voltage for data transmission, i.e. the signal was determined by the voltage difference between the two terminals of the circuit, then in the HART protocol ( Highway Addressable Remote Transducer) the electrical signal is current. HART networks are built on the principle of an analog current loop with frequency modulation of the signal.
The HART protocol is capable of communicating at speeds up to 1200 Baud. A diagram explaining the operation of devices using the HART protocol is shown in Fig. 5.
Rice. 5.
HART uses one full cycle of 1200 Hz to transmit a logical 1, and two partial cycles of 2200 Hz to transmit a logical 0.
As can be seen in Figure 5, the HART component is superimposed on the 4...20 mA current loop. Since the average value of the sine wave over the period is “0”, the HART signal has no effect on the analog 4...20 mA signal.
The HART protocol is based on the master-slave principle, that is, the field device responds to the system's request. The protocol allows for two control devices (control system and communicator).
There are two modes of operation of sensors that support data exchange via the HART protocol.
In the mode of transmitting digital information simultaneously with an analog signal, the sensor operates in analog automated process control systems, and exchange via the HART protocol is carried out via a HART communicator or computer. In this case, you can remotely (distance up to 3000 m) complete setup and configuration of the sensor.
In multipoint mode, the sensor transmits and receives information only in digital form. The analog output is automatically fixed at the minimum value (device power supply only - 4 mA) and does not contain information about the measured value. Information about process variables is read using the HART protocol.
Up to 15 sensors can be connected to one pair of wires. Their quantity is determined by the length and quality of the line, as well as the power of the sensor power supply. All sensors in multipoint mode have their own unique address from 1 to 15, and each one is addressed to the corresponding address. The communicator or control system detects all sensors connected to the line and can work with any of them.
DS8500
Maxim Integrated Products, Inc. introduced DS8500 - single-chip HART modem that meets the requirements of the HART specification at the physical level.
As can be seen in Fig. 6, a modulator and demodulator of 1200/2200 Hz frequency-modulated signal are integrated on the chip.
Rice. 6.
The chip has very low power consumption and, thanks to the implemented digital signal processing, requires only a few external components. The input signal is digitized by the ADC and sent to a digital filter/demodulator. The modem architecture allows reliable signal detection even in noisy environments. The output DAC generates a sinusoidal voltage and maintains a phase shift when switching between 1200 and 2200 Hz. Low consumption is achieved by inhibiting the operation of the receiver during signal transmission, the transmitter does not operate during reception. The DS8500 is ideal for creating low-power process control system transmitters.
As can be seen in Fig. 7, just a few external components and a 20-pin TQFN miniature package reduce the cost and size of the product.
Rice. 7.
Main features of the modem:
- Single-chip solution for half-duplex transmission, 1200 baud, FSK modulation and demodulation;
- Digital signal processing, providing reliable detection of the input signal in a noisy environment;
- Sinusoidal output signal with minimal harmonic distortion;
- Standard clock frequency 3.6864 MHz;
- Compliance with the requirements of the HART specification at the physical level;
- Supply voltage in the range 2.7…3.6V;
- Maximum current consumption 285 µA;
- Miniature 20-pin TQFN package with dimensions 5x5x0.8mm.
Thanks to the active use of the HART protocol, the DS8500 chip is indispensable when developing transmitters for data acquisition devices (temperature, pressure, etc.), HART modems or HART multiplexers.
Conclusion
Although the RS-232 and RS-485 standards were created more than 30 years ago, they are still actively used. Previously, no personal computer could do without a COM port, data transfer through which is based on the RS-232 protocol. Even though in modern computers the COM port has long been replaced by more modern ones, this does not mean that the RS-232 and RS-485 protocols are forgotten.
In industrial networks they have no equal due to the high stability and long distances over which data transmission is ensured. However, this reliability is determined not only by the initial successful development of the protocol, but also by the constant improvement of the hardware.
Interface products from MAXIM ideally meet the needs of the Russian industrial electronics market, and the interfaces will live for a long time. Maxim is actively improving the reliability characteristics of communication chips and expanding their additional functionality.
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Maxim acquired Teridian
Company Maxim announced the acquisition of Teridian Semiconductor Corporation. Teridian Semiconductor is a fabless company headquartered in Irvine, California. The company is a major supplier of semiconductor components, with a primary focus on chips for energy meters and smart energy systems. It supplies three of the four major energy meter manufacturers in the United States and over fifty meter manufacturers worldwide. The key differentiator of Teridian smart sensors is their new architecture, which allows for more accurate power measurements over a wider dynamic range. To optimize time to market and reduce costs, energy meter manufacturers require ICs with high on-chip integration and turnkey multi-layer solutions. Maxim's demonstrated ability to integrate multiple signal functions will be extremely useful in the production of highly integrated system-on-chip (SoC) and turnkey solutions that meet these requirements. It has been stated that the number of smart meters using both system-on-chip and off-the-shelf solutions should increase by 10% annually until 2014.
As Maxim CEO Tunk Doluca recently noted ( Tunc Doluca): “Investment in global smart energy systems will need to be significant in order to use power plants and energy networks more efficiently. Energy metering and network communications are key components of a smart energy system, and therefore inevitably lead to the development of new energy meters to replace outdated ones. The acquisition of Teridian's product line and team will greatly accelerate our penetration into this fast-growing market and help us strengthen our position."
The RS-422 and RS-485 interfaces eliminate the shortcomings of the RS-232 interface, which is widely used in personal computers. The design of RS-422/RS-485 interfaces is based on the principle of differential data transmission. Its essence is to transmit one signal over two wires twisted together to form a twisted pair. Usually one wire is conventionally referred to as 'A', and the other - 'B'. The useful signal is the potential difference between wires A and B: U A – U B = U AB. To organize the interfaces, linear transmitters with differential outputs and linear receivers with differential inputs are required.
In Fig. Figure 1 shows a conventional image of a linear transmitter of the RS-422/RS-485 interfaces and a timing diagram of its output signal. The transmitter outputs 2 to 6 V between terminals A and B. The transmitter also has a circuit common point (wire) terminal C. Unlike the RS-232C interface, the common wire is not used here to determine the state of the data line, but is used only to connect the signal ground. If the transmitter output is 2< U AB < 6 В, то это соответствует логическому 0, а диапазон -6 < U AB < -2 В соответствует логической 1.
Rice. 1. RS-422/RS-485 interface transmitter:
a) - symbol; b) - timing diagram of the output signal U AB
The linear transmitter of the RS-485 interface must have a “Resolution” control signal input. The purpose of this signal is to connect the transmitter outputs to line pins A and B. If the Enable signal is in the Off state (usually logic 0), then the transmitter will be disconnected from the line. The shutdown state of a line transmitter is commonly referred to as its third or Z-state.
A differential receiver analyzes signals from the communication line arriving at its inputs A and B. If at the receiver input U A – U B = U AB > 0.2 V, then this corresponds to logical 0, if U A – U B< -0,2 В, то это логическая 1. Диапазон | U A – U B | < 0,2 В является зоной нечувствительности (гистерезисом), защищающей от воздействия помех. Линейный приемник также должен иметь вывод C общего провода схемы, чтобы выполнить сигнальное заземление.
The use of a differential signal transmission method ensures good noise immunity of the interfaces. For hardware implementation of the interface, transceiver chips (transceivers) with differential inputs/outputs connected to the line and digital inputs/outputs connected to the microcontroller UART module are used.
Comparison of RS-422 and RS-485 interfaces. The standard defines RS-422 as a point-to-point interface with one transmitter and up to ten receivers. In Fig. Figure 2 shows a diagram of connecting devices to interface lines for simplex (one-way) exchange. For duplex exchange, you need a second pair of wires with the same connection of devices.
Rice. 2. Connecting devices to the RS-422 interface communication line
The standard defines RS-485 as a multidrop interface that allows up to 32 transmitters, receivers, or combinations thereof to be connected to a single line. In Fig. Figure 3 shows a diagram of connecting devices to interface lines for half-duplex exchange. The differential inputs of the RS-422/485 interface receivers protect against interference, but in this case the common points C of the devices must be connected to each other and to the grounding bus. When the communication line is long, an additional third interface wire is used to connect common points. If shielded twisted pair is used, the shield can be used as the third wire.
Rice. 3. Connecting devices to the RS-485 interface communication line
Matching resistance in the communication line. At long distances between twisted-pair devices and high transmission speeds, the so-called long line effects begin to appear. In this case, the transmitted signals are distorted due to the reflection of signals at the ends of the communication line.
It is known that any electrical communication line is characterized by wave impedance, which is determined only by its parameters: the area and shape of the cross-section of the wires, their relative position, the thickness and type of dielectric between them. If you connect a resistor with a resistance equal to the wave resistance to the end of the line, the signal will not be reflected from it. Such a line is called consistent. Distortion in it is minimal. The matching resistor R C is installed at the end of the line towards which the signal is transmitted. In the RS-422 interface it is located at the end of the line opposite from the transmitter (see Fig. 2). In the RS-485 interface, if the transmission is in two directions, terminating resistors RC are installed at both ends of the communication line (see Fig. 3). The twisted pairs currently used have a characteristic impedance of about 120 Ohms, so the resistance of the matching resistors is also taken to be 120 Ohms. The term "matching" resistor is not generally accepted. Often the terms used instead are: end-of-line or terminal resistor.
The maximum data transfer rate over RS-422/RS-485 interfaces is determined by many factors: the length and parameters of the communication line, parameters of receivers and transmitters. The maximum transmission speed over short distances (up to 12 m) is limited by the speed of the transmitters and is equal to 10 Mbit/s according to the standard. At medium distances (tens and hundreds of meters), the transmission speed decreases due to an increase in losses in the cable insulation capacitances and the active resistance of the wires. For example, with a line length of 120 m, the maximum transmission speed does not exceed 1 Mbit/s. The maximum length of the communication cable according to the standard is limited to 1200 m, while the transmission speed does not exceed 100 Kbit/s.
The advantages of the RS-422 and RS-485 interfaces are: low cost of connecting cables; low cost of selling transceivers; a large fleet of operating equipment that implements these standards; possibility of organizing galvanic isolation.
The disadvantage of interfaces is that they are not included in the standard configuration of computers and microcontrollers. The interfaces have fairly significant power consumption and a relatively low data transfer rate.
Share to:The EIA RS232C standard interface is designed for serial communication of two
devices. It is generally accepted and widely used in hardware systems with
connecting external equipment to a personal computer. Interface
RS/232C involves the use of “single-ended” transmitters and
receivers, while data transmission is carried out using “asymmetrical”
signal along two lines - ТхD and RxD, and the signal amplitude is measured relative to the line
GND (“zero”). A logical unit corresponds to a range of amplitude values
signal (voltage) from –12 to –3 V, logical zero – from +3 to +12 V. Range from
–3 to +3 V corresponds to the dead zone, which determines the hysteresis of the receiver.
The asymmetry of the signal causes low noise immunity of this
interface, especially with industrial interference. Availability of receive (RxD) and transmit lines
(TxD) data allows you to support full-duplex information transmission, i.e.
information can be both transmitted and received at the same time.
Advantages - simplicity.
Disadvantages - only one device is connected to one port, the signal transmission range without additional gadgets is only a few meters
Hardware is the most widely used method for data flow control.
management. For correct data transmission, it is necessary that the receiver is in
state of readiness to receive information. With hardware control method
The RTS/CTS signal is used to stop data transmission if
the receiver is not ready to receive them. Hardware flow control provides the most
quick response of the transmitter to the state of the receiver.
When designing industrial automation systems, the greatest
information networks based on the standard interface have become widespread
EIA RS485. Unlike RS/232, this interface provides data transmission from
using a “symmetrical” (differential) signal on two lines (A and B)
(see figure) and the use of an additional line for potential equalization
grounding of devices connected to an RS/485 network. Logical signal level
determined by the voltage difference on the lines (A - B), with a logical unit
corresponds to a range of voltage values from +0.2 to +5 V, and to logical zero – a range
values from –0.2 to –5 V. The range from –0.2 to +0.2 V corresponds to the dead zone
receiver When using this interface, the maximum length of the communication line between
extreme devices can be up to 1200 m. Moreover, in the most remote
It is recommended to install terminal terminating resistors at network points from each other
(terminators) that allow you to compensate for the characteristic impedance of the cable and
minimize the amplitude of the reflected signal.
The resistance of the matching resistors depends on the length of the line and the number of devices. It should be in the range from 100 to 620 ohms.
Both of these interfaces support asynchronous transfer mode. Data
are sent in blocks (frames), the format of which is shown in Fig. 1.2. Transfer of each
frame begins with a start/bit, signaling the receiver about the start of transmission, for
followed by data bits and a parity bit. Completes the sending of a stop/bit, guaranteeing
pause between sendings.
For asynchronous mode, a number of standard exchange rates are adopted: 50, 75, 110, 150,
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 bps. Number of data bits
can be 5, 6, 7 or 8 (5/ and 6/bit formats are not very common).
The number of stops/bits can be 1, 1.5 or 2 (“one and a half bits” means only
duration of the stop interval). 
Chapter.