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EIA/TIA-232, 422, and 485 Networks

Some of the simplest types of digital communication networks found in industry are defined by the EIA (Electronic Industry Alliance) and TIA (Telecommunications Industry Alliance) groups, under the numerical labels 232, 422, and 485. This section discusses these three network types.

 

EIA/TIA-232

The EIA/TIA-232C standard, formerly1 known as RS-232, is a standard defining details found at layer 1 of the OSI Reference Model (voltage signaling, connector types) and some details found at layer 2 of the OSI model (asynchronous transfer, “handshaking” signals between transmitting and receiving devices). In the early days of personal computers, almost every PC had either a 9-pin or a 25-pin connector (and sometimes multiple of each!) dedicated to this form of digital communication. For a while, it was the way peripheral devices such as keyboards, printers, modems, and mice were connected to the PC. USB (Universal Serial Bus) has now all but replaced EIA/TIA-232 for personal computers, but it still lives on in the world of industrial devices.

EIA/TIA-232 networks are point-to-point, intended to connect only two devices2. The signaling is single-ended (also known as unbalanced), which means the respective voltage pulses are referenced to a common “ground” conductor, a single conductor used to transfer data in each direction:


EIA/TIA-232 Networks are point-to-point, intended to connect only two devices.

EIA/TIA-232 specifies positive and negative voltages (with respect to the common ground conductor) for its NRZ signaling: any signal more negative than -3 volts detected at the receiver is considered a “mark” (1) and any signal more positive than +3 volts detected at the receiver is considered a “space” (0). EIA/TIA-232 transmitters are supposed to generate -5 and +5 volt signals (minimum amplitude) to ensure at least 2 volts of noise margin between transmitter and receiver.

Cable connectors are also specified in the EIA/TIA-232 standard, the most common being the DE-93 (nine-pin) connector. The “pinout” of a DE-9 connector for any DTE (Data Terminal Equipment) device at the end of an EIA/TIA-232 cable is shown here:

 

DE-9 Cable Connector with its pin numbering shown
Pin number Assignment Abbreviation
1 Carrier Detect CD
2 Received Data RD
3 Transmitted Data TD
4 Data Terminal Ready DTR 
5 Signal Ground Gnd
6

    Data Set Ready

DSR
7 Request To Send RTS
8 Clear To Send CTS
9

     Ring Indicator

RI

Those terminals highlighted in bold font represent those connections absolutely essential for any EIA/TIA-232 link to function. The other terminals carry optional “handshaking” signals specified for the purpose of coordinating data transactions (these are the layer 2 details).

For DCE (Data Communications Equipment4) devices such as modems, which extend the EIA/TIA-232 signal path onward to other devices, the assignments of pins 2 and 3 are swapped: pin 2 is the Transmitted Data (TD) pin while pin 3 is the Received Data (RD) for a DCE device. This allows straight pin-to-pin connections between the DTE and DCE devices, so the transmit pin of the DTE device connects to the receive pin of the DCE, and visa-versa.

 

For DCE (Data Communications Equipment) devices such as modems, which extend the EIA/TIA-232 signal path onward to other devices, the assignments of pins 2 and 3 are swapped.
 

isa-v If one desires to directly connect two DTE devices together using EIA/TIA-232, a special cable called null modem must be used, which swaps the connections between pins 2 and 3 of each device. A "null modem" connection is necessary for the transmit pin of each DTE device to connect to the receive pin of the other DTE device:

 

Two DTE devices are connected and able to communicate with the use of null modem cable.

The concept of a “null modem” is not unique to EIA/TIA-232 circuits5. Any communications standard where the devices have separate “transmit” and “receive” channels will require a “null modem” connection with transmit and receive channels swapped to be able to communicate directly without the benefit of interconnecting DCE devices. Four-wire EIA/TIA-485 and Ethernet over twisted-pair wiring are two examples of network standards where a “null” style cable is required for two DTE devices to directly connect.

EIA/TIA-232 networks may be simple, but they tend to be rather limited both in data bit rate and distance, those two parameters being inversely related. References to the EIA/TIA-232 standard repeatedly cite a maximum data rate of 19.2 kbps at 50 feet cable rate. Experimental tests6 suggest greater rate/distance combinations may be possible in optimum conditions (low cable capacitance, minimum noise, good grounding). Since this communications standard was developed to connect peripheral devices to computers (typically within the physical span of one room), and at modest speeds, neither of these limitations were significant to its intended application.


 

EIA/TIA-422 and EIA/TIA-485

The next two network standards7 are less comprehensive than EIA/TIA-232, specifying only the electrical characteristics of signaling without any regard for connector types or any layer 2 (handshaking) considerations. Within these domains, the 422 and 485 standards differ significantly from 232, their designs intended to optimize both maximum cable length and maximum data rate.

To begin with, the electrical signaling used for both EIA/TIA-422 and EIA/TIA-485 is differential rather than single-ended (balanced rather than unbalanced). This means a dedicated pair of wires is used for each communications channel rather than a single wire whose voltage is referenced to a common ground point as is the case with EIA/TIA-232:


Differential Transceivers

Using dedicated wire pairs instead of single conductors sharing a common ground means that EIA/TIA-422 and EIA/TIA-485 networks enjoy much greater immunity to induced noise than EIA/TIA-232. Any electrical noise induced along the length of network cables tends to be fairly equal on all non-grounded conductors of that cable, but since the receivers in EIA/TIA-422 and EIA/TIA-485 networks response only to differential voltages (not common-mode voltages), induced noise is ignored.

The advantage differential signaling enjoys over single-ended signaling may be understood by graphical comparison. The first illustration shows how electrical noise imposed on the ungrounded conductor of a simplex communications cable becomes superimposed on the digital data signal, detected at the receiving end. Noise is modeled here as a voltage source in series along the ungrounded conductor, near the receiving end. In reality, it is more likely to be distributed along the bulk of the cable length:

 

Electrical noise imposed on the ungrounded conductor of a simplex communications cable becomes superimposed on the digital data signal, detected at the receiving end.

If the superimposed noise voltage detected at the receiver has sufficient peak-to-peak amplitude to push the signal voltage above or below critical threshold levels, the receiver will interpret this as a change of digital state and cause corruptions in the data stream.

By contrast, any noise superimposed on ungrounded conductors in a differential signaling circuit cancel at the receiver, because the close proximity of those two conductors ensures any induced noise will be the same. Since the receiver responds only to differential voltage between its two inputs, this common-mode noise cancels, revealing a “clean” data signal at the end:

 

Receiver responds only to differential voltage between its two inputs.

 

Both EIA/TIA-422 and EIA/TIA-485 systems use differential signaling, allowing them to operate over much longer cable lengths at much greater cable speeds than EIA/TIA-232 which is single-ended. Other high-speed network standards including Ethernet and USB (Universal Serial Bus) use differential signaling as well.

EIA/TIA-422 is a simplex (one-way) communications standard, whereas EIA/TIA-485 is a duplex (two-way) standard. Both support more than two devices on a network segment. With EIA/TIA-422, this means one transmitter and multiple receivers. With EIA/TIA-485, this may include multiple transceivers (devices capable of both transmitting and receiving at different times:half-duplex). Four wires are necessary to connect two such devices when full-duplex (simultaneous two-way communication) is required, and full-duplex is only practical between two devices (as shown in the previous illustration).

EIA/TIA-422 and EIA/TIA-485 specify positive and negative voltage differences (measured between each dedicated wire pair) for its signaling: any signal more negative than -200 millivolts is a “mark” (1) and any signal more positive than +200 millivolts is a “space” (0). These voltage thresholds are much lower than for EIA/TIA-232 (} 3 volts) due to the noise-canceling properties of differential signaling. EIA/TIA-422 transmitters (“drivers”) are supposed to generate -2 and +2 volt signals (minimum amplitude) to ensure at least 1.8 volts of noise margin between transmitter and receiver. EIA/TIA-485 drivers are allowed a smaller noise margin, with the minimum signal levels being -1.5 volts and +1.5 volts.

The maximum recommended cable length for both EIA/TIA-422 and EIA/TIA-485 networks is 1200 meters, which is greater than half a mile8. The maximum data rate is inversely dependent on cable length (just as it is for EIA/TIA-232), but substantially greater owing to the noise immunity of differential signaling. With the long cable lengths and higher data rates made possible by differential signaling, some applications may require terminating resistors to eliminate reflected signals. Experiments conducted by Texas Instruments demonstrate acceptable signal integrity at 200 kbps over a cable 100 feet long with no termination resistors. With a termination resistor at the receiver input (for simplex data transmission) in place on the same 100 foot cable, a data rate of 1 Mbps was achieved.

Due to the lack of standardization for cable connectors in EIA/TIA-422 and EIA/TIA-485 networks, there are no established pin numbers in certain connectors designated for the differential transmit and receive conductors. A common convention seen in industrial devices, though, are the labels “A” and “B”, alternative labeled “-” and “+” or “A-” and “B+” in honor of their idle-state polarities (the “mark” or “1” state). In a 4-wire EIA/TIA-485 network, where full-duplex operation is possible, the terminals and connections will look something like this:

 

EIA/TIA-486 Full-Duplex Connections and Terminals will look like this.

Note the use of a ground conductor connecting both devices together. Even though the data signaling is differential and therefore does not theoretically require a common ground connection (since common-mode voltage is ignored), a ground connection helps ensure the common-mode voltage does not become excessive, since real receiver circuits will not properly function when exposed to certain levels of common-mode voltage.

A popular connection scheme for EIA/TIA-485 half-duplex operation is where the Transmitted Data (TD) and Received Data (RD) terminal pairs are combined, so that two-way communication may occur over one pair of wires. With such devices, it is customary to label the terminals simply as “Data” (A- and B+):

 

A popular connection scheme for EIA/TIA-485 half duplex operation where the Transmitted Data (TD) and Received Data (RD) terminal pairs are combined so that two-way communication may occur over one pair of wires.

The possibility of half-duplex operation begs the question of channel arbitration and device addressing, but since the EIA/TIA-485 standard does not specify anything outside of layer 1 concerns, these matters are left to other networking standards to fulfill. In other words, EIA/TIA- 485 is not a complete data communications standard, but merely serves as the layer 1 component of other standards such as Allen-Bradley’s Data Highway (DH), Opto 22’s Optomux, and others.

Given the potential for high-speed communication along lengthy runs of cable using EIA/TIA-422 or EIA/TIA-485, the potential necessity of terminating resistors to prevent signal “reflection” is very real. Networks operating with short cables, and/or slow data rates, may work just fine without termination resistors9. However, the effects of reflected signals grows more pronounced as the reflection time (time-of-flight for the signal to travel “round-trip” from one end of the cable to the other and back) approaches a substantial fraction of the bit time.

No network should have more than two termination resistors, one at each (far) end, and care should be taken to limit the lengths of all cable “stubs” or “spurs” branching off of the main “trunk” cable:

 


No network should have more than two termination resistors, one at each (far) end, and care should be taken to limit the lengths of all cable “stubs” or “spurs” branching off of the main “trunk” cable

 

The proper value for these resistors, of course, is equality with the characteristic impedance10 of the cable itself. A termination resistor value greater than the cable’s surge impedance will still allow positive reflections of limited amplitude, while a termination resistor value less than the cable’s surge impedance will still allow negative reflections of limited amplitude.

However, the inclusion of resistive loads to an EIA/TIA-422 or EIA/TIA-485 network may cause other problems. Many devices use a pair of biasing resistors internally to establish the “mark” state necessary for idle conditions, connecting the “A” terminal to the negative supply voltage rail through a resistor and the “B” terminal to the positive supply voltage rail through another resistor. Connecting a terminating resistor between terminals “A” and “B” will alter the voltage levels normally provided by these biasing resistors, consequently causing problems. 

The following schematic diagram shows the equivalent circuit of an EIA/TIA-485 transceiver device, with and without a terminating resistor connected:


Equivalent circuit of an EIA/TIA-485 transceiver device

When the driver is in high-impedance (High-Z) mode, the “idle” state of the wire pair will be established by the bias resistors (equal to the supply voltage so long as there is no loading). However, a terminating resistor will act as a DC load to this biasing network, causing a substantial reduction of the “idle” state voltage toward 0 volts. Recall that -200 millivolts was the receiving threshold value for a “mark” state in both EIA/TIA-422 and EIA/TIA-485 standards (terminal “A” negative and terminal “B” positive). If the presence of a terminating resistor11 reduces the idle state voltage to less than 200 millivolts absolute, the network’s function may be compromised.

Thus, we see that the inclusion of any terminating resistors must be accompanied by an analysis of the devices’ bias resistor networks if we are to ensure robust network operation. It is foolhardy to simply attach terminating resistors to an EIA/TIA-422 or EIA/TIA-485 network without considering their combined effect on biasing.



1The designation of "RS-232" has been used for so many years that it still persists in modern writing and manufacturers' documentation, despite the official status of the EIA/TIA label. The same is true EIA/TIA-422 and EIA/TIA-485, which were formerly known as RS-422 and RS-485, respectively.

2"Daisy-chain" networks formed of than two devices communicating via EIA/TIA-232 signals have been built, but they are rarely encountered, especially in industrial control applications.

3Often (incorrectly) called a "DB-9" connector

4Also known by the unwieldy acronym DCTE (Data Circuit Terminating Equipment). Just think of “DTE” devices as being at the very end of the line, whereas “DCE” devices are somewhere in the middle, helping to exchange serial data between DTE devices.

5In fact, the concept is not unique to digital systems at all. Try talking to someone using a telephone handset held upside-down, with the speaker near your mouth and the microphone hear your ear, and you will immediately understand the necessity of having “transmit” and “receive” channels swapped from one end of a network to the other!

6Once I experimented with the fastest data rate I could “push” an EIA/TIA-232 network to, using a “flat” (untwisted, unshielded pair) cable less than ten feet long, and it was 192 kbps with occasional data corruptions. Park, Mackay, and Wright, in their book Practical Data Communications for Instrumentation and Control document cable lengths as long as 20 meters at 115 kbps for EIA/TIA-232, and 50 meters (over 150 feet!) at 19.2 kbps: over three times better than the EIA/TIA-232 standard.

7Former labels for EIA/TIA-422 and EIA/TIA-485 were RS-422 and RS-485, respectively. These older labels persist even today, to the extent that some people will not recognize what you are referring to if you say “EIA/TIA-422” or “EIA/TIA-485.”

81200 meters is the figure commonly cited in technical literature. However, Park Mackay, and Wright, in their book Practical Data Communications for Instrumentation and Control document EIA/TIA-422 and EIA/TIA-485 networks operating with cable lengths up to 5 km (over 16,000 feet) at data rates of 1200 bps. Undoubtedly, such systems were installed with care, using high-quality cable and good wiring practice to minimize cable capacitance and noise.

9In fact, a great many EIA/TIA-485 networks in industry operate “unterminated” with no problems at all.

10For detailed explanation of how and why this is necessary, refer to AC Electricity: Transmission Lines

11Actually two terminating resistors in parallel, since one will be at each end iof the cable! The actual DC biasing network will be more complicated as well if more than one device has its own set of internal bias resistors.

 

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