EIA-485

EIA-485 , also known as RS-485 , is an industry standard for a physical interface for asynchronous serial data transmission . The symmetrical line increases the electromagnetic compatibility .

General

Circuit diagram of an EIA-422 / EIA-485 receiver

Circuit diagram of an EIA-422 / EIA-485 transmitter

EIA-485 uses a pair of lines to carry the inverted and a non-inverted level of a 1-bit data signal. At the receiver, the original data signal is reconstructed from the difference between the two voltage levels. This has the advantage that common-mode interference does not affect the transmission and thus the immunity to interference is increased. In contrast to EIA-232 , much longer transmission paths and higher clock rates are possible. Compared to the EIA-422 standard, the transmitters have short-circuit-proof output stages thanks to an integrated resistor so that sending two transmitters in the opposite direction does not lead to defects. In addition, several transmitters and several receivers can be connected to one wire pair ("multipoint").

In contrast to other buses, the EIA-485 only defines the electrical interface conditions. The protocol can be selected application-specifically. Therefore, EIA-485 devices from different applications or manufacturers will i. General do not understand. If data is to be transported, the Universal Asynchronous Receiver Transmitter Protocol ( UART ), known from RS-232 interfaces, is often used for character transmission . Usually eight equivalent bits per frame are transmitted here.

It is not defined how the driver is released. Control by RTS is very widespread , both with EIA-232- EIA-485 converters and in the firmware in USB- EIA-485 converters based on CDC . Activation through send data and fallback after a timeout is also common. Since the correct amount of time depends directly on the bit rate, its value is critical. USB-EIA-485 converters “know” the end of the send data and are therefore easier to use.

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It is named after the Electronic Industries Alliance (EIA), which previously issued its standards with the designation RS for Radio Sector . With the expansion to include areas beyond radio technology, the prefix was retained, but was no longer officially an abbreviation. Today RS is mostly read as the Recommended Standard . The standard is now administered by the Telecommunications Industry Association (TIA for short), has the designation TIA-485-A and the title Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (ANSI / TIA / EIA-485-A- 98) (R2003) . The last part of the name says that the standard was confirmed in 2003 without any technical changes.

technology

The symmetrical line of the EIA-485 interface works on the transmitter side with signal voltages of at least ± 1.5 V, at most ± 6 V. Typically, the driver uses a bridge circuit so that the signal amplitude is just below the operating voltage of e.g. B. 5 V or 3.3 V. The switching threshold of the receiver must be in the range of ± 0.2 V. In contrast to the mass-based EIA-232 interface or the old TTY - current interface of the Telegraph is a 485 recipients compared to the symmetrical structure of the signal conductor common mode noise largely unaffected.

As a rule, the interface only uses one wire pair and is operated half-duplex , with two wire pairs full-duplex operation is possible. The connection is multipoint-capable, i.e. up to 32 participants (or unit loads, a hypothetical load size) can be connected to the EIA-485 bus. There are transceiver modules that represent ½, ¼ or even just ⅛ unit load. Compared to a normal transceiver, these components then have twice, four times or eight times the input resistance . Networks with up to 256 participants (⅛ Unit Load) can be set up with this. Cable lengths of up to 1.2 km (4000 ft ) and transmission rates of up to 10 Mbit / s are usually supported, with the maximum transmission rate only being achieved with cable lengths of up to 12 m. However, there are also solutions for up to 320 participants and for transmission rates of up to 100 Mbit / s. The actual possible maximum network size and the maximum transmission rate also depend heavily on the structure of the network. Star topologies are not provided, the structure in chain form ( daisy chain ) is common.

Terminating resistor with bias network

Signal curve with bias network

Since the EIA-485 interface is a bus system (in contrast to the point-to-point connection with the EIA-232), the cable ends should be terminated (at least for longer cable lengths or higher transmission rates) . As a rule, a passive termination is used by connecting the signal lines via a 120 Ω resistor at both ends of the bus. An optional bias network (the 720 ohm resistors in the picture) improves the signal-to-noise ratio in the case of inactive line drivers by using a voltage other than zero, which is otherwise only given by the hysteresis of the receiver.

In the case of long cable lengths or deviating ground potentials, there may be greater potential differences between the bus participants that are above the common-mode rejection and therefore hinder communication. This can be improved by carrying the earth line and avoided by galvanic separation of the participants from their device earth or local earth electrodes.

EIA-485 only specifies the electrical properties of the interface, it does not define any protocol or pin assignment. This is why there is no uniform pin assignment of an EIA-485 connector, so that when using different EIA-485 devices, the device documentation must always be observed. With Profibus , which is based on the EIA-485 standard, for example, pins 3 and 8 of 9-pin D-Sub plugs and sockets are used for the data line.

EIA-485 is also widespread in the checkout area, where IBM printers and other peripheral devices are connected via proprietary plug-in connections.

The DMX interface used in event technology for lighting control is based on EIA-485.

Overview of the most important specifications

parameter	value
Number of recipients	32 recipients
Maximum cable length	1200 m
Maximum data transfer rate	12 Mbps
Common mode input voltage	−7 V to +12 V
Input resistance of the receiver	12 kΩ (1 unit load)
Input sensitivity of the receiver	± 200 mV

Transmitter
DI	DE	Y	Z
H	H	H	L.
L.	H	L.	H
x	L.	Z	Z

Receiver
A - B	RE	RO
U _ID ≥ 0.2 V	L.	H
0.2 V> U _ID > −0.2 V	L.	n. def.
U _ID ≤ −0.2 V	L.	L.
x	H	Z
Open	L.	n. def.

The non-inverted line is usually marked with B or + or ..P (positive), the inverted one with A , - or ..N . With two bus lines this would be e.g. B. + and - or RXTX-P and RXTX-N , with four bus lines corresponding to TX + , TX− , RX + , RX− . Mass is z. B. SG for Signal Ground .

EIA-485 and EIA-422

Due to the great similarity, EIA-485 and EIA-422 are often used synonymously, EIA-422 is viewed as a subset of the EIA-485 standard. However, both standards are not 100% compatible with each other. EIA-485 components can be used in EIA-422 networks without problems, but not vice versa. Important differences are:

EIA-422 components usually do not have a driver enable with which the transmitter can be switched off. In a network with several drivers, EIA-422 components cannot be used, as otherwise two drivers may drive against each other. EIA-422 components are not protected against this.
In the case of EIA-422 components that do have a driver enable, the driver strength is not sufficient to drive a network terminated at both ends.
The common mode dielectric strength of the EIA-422 drivers is not as great as that of the EIA-485 receivers. If common-mode voltages or offsets occur in networks with several drivers that are still permitted for the receiver, the maximum permitted voltages on the driver may have already been exceeded.

In an EIA-485 network, EIA-422 components should therefore only be used as receivers. However, it must be noted that an EIA-422 receiver with its 4 kΩ input resistance corresponds to 3 unit loads.

Difference to EIA-232

The now obsolete interface EIA- / RS-232 (point-to-point connection), in contrast to the newer standards EIA- / RS-422 (one transmitter, several receivers) and EIA- / RS-485 (several transmitters, several receivers) different level ratios. While the RS-232 is designed to be almost pseudo-differential (signal reference is GND for all signals in both directions), EIA-485 is designed to be clearly differential . In order to be able to tolerate the intermodulation between the different bidirectional signals of the RS-232, the RS-232 hub and switching thresholds were increased. Corresponding interface ICs for RS-232 (e.g. MAX232) implement an additional, integrated auxiliary voltage generator (e.g. ± 12 volts).

With the newer interfaces (EIA-422, EIA-485), however, it was recognized that the quality of a symmetrical signal transmission is completely independent of the level of the switching threshold (middle potential). If the switching threshold is simply placed in the middle between VDD and GND (+5 volts against 0 volts), a transmission quality that is superior to that of the EIA-232 is achieved with conventional BiCMOS modules.

While the minimal form of RS-232 can consist of a transmit signal TX, a receive signal RX and a ground (GND), RS-485 manages with two signals A and B, which are pushed together once in one direction and then in the other Transport data. The resulting risk of two partners sending messages back causes a slightly higher circuit complexity.

This disadvantage is offset by serious advantages. On the one hand, the generation of positive and negative auxiliary voltages is completely omitted with RS-422 / -485, which significantly reduces the number of components, on the other hand, the now smaller signal swings allow a considerable reduction in power loss, since an adaptation to the line impedance (independent of the signal swing) is always required is. As a side effect, with the same displacement current, there is an additional gain in speed; with a smaller stroke and the same edge steepness, the level has reached the final value more quickly. This means that the transferable data rate or distance can be increased considerably despite the reduced power loss. ${\ displaystyle \ left (\ textstyle {\ frac {I} {C}} = {\ frac {\ mathrm {d} u} {\ mathrm {d} t}} \ right)}$

Since sending to the opposite direction must be avoided even with two-point connections via communication protocol, RS-485 actually implies the possibility of forming multipoint networks without additional circuitry.

literature

Gerhard Schnell and Bernhard Wiedemann: Bus systems in automation and process technology, Vieweg + Teubner Verlag, Wiesbaden 2008, ISBN 978-3-8348-0425-9 .