How RS-232, RS-422 and RS-485 Networks are applied in Instrumentation

RS-232

Presently, this standard is known as TIA-232C. These are point-to-point networks with a purpose of connecting only two devices. The signaling is single-ended (unbalanced) which implies that the respective voltage pulses are referenced to a common ground conductor and a single conductor is used to transfer data in each direction as illustrated in figure 1(a)

RS-232 (TIA-232) specifies positive and negative voltages with respect to ground for is non-return-zero (NRZ) signaling i.e. 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). RS-232 (TIA-232) transmitters are supposed to generate -5 and +5 volt signals as the minimum amplitude to ensure at least 2 volts of noise margin between the transmitter and receiver.

The cable connectors are specified in the TIA-232 standard e.g. the DE-9 (9-pin) connector. The pinout of the DE-9 connector for a DTE (Data terminal equipment) device at the end of a RS-232/TIA-232 cable is illustrated in the figure below:

Table 1 Pinout of the DE-9 Connector

The terminals 2, 3 and 5 (with asterisk) represent the connections absolutely essential for any RS-232/TIA-232 link to work. The other terminals carry optional ”handshaking” signals specified for the intention of coordinating data transactions.

Note, data communication equipment (DCE) like modems that extend the RS-232/TIA-232 signal path, onward toward other devices have the assignments of pin 2 and pin 3 swapped i.e. pin 2 is the transmitted data (TD) pin whereas pin 3 is the received data (RD) pin for a given DCE equipment. This permits straight pin-to-pin connections between the data terminal equipment (DTE) and data communication equipment (DCE) devices as shown below:

To connect directly two DTE devices, together using RS-232/TIA-232, a special cable called null modem must be used, which swaps the connections between pin 2 and pin 3 of each devices. The null modem connection is required for the transmit pin of each of the DTE device to connect to the receiver pin of the other DTE device as shown below in figure 1(c).

RS-232/TIA-232 networks though simple, are limited in data bit rate and distance. TIA-232 standard mentions a maximum data rate of 19.2 kbps at 50 feet cable rate. Nevertheless, experimental tests suggest higher rate/distance combinations may be possible in optimum conditions i.e. where we have low cable capacitance, good grounding and minimum noise.

Related: Basic Features of Foundation Fieldbus (FF)

RS-422 and RS-485

RS-422 standard is also called TIA-422 and RS-485 is also termed to as TIA-485.

The electrical signaling employed for both RS-422 and RS-485 is differential rather than single ended i.e. it is balanced rather than unbalanced as the case with RS-232. This implies that, a dedicated pair of wires is used for each communication rather than a single wire whose voltage is referenced to a common ground as the case with RS-232/TIA-232. This is illustrated in figure 1(d) below:

The use of dedicated wire pairs instead of single conductors sharing a common ground enables RS-422/TIA-422 and RS-485/TIA-485 networks to have much greater immunity to induced noise than RS-232/TIA-232. Any noise that is induced along the length of the cables tends to be relatively equal on all non-grounded conductors of that cable but since the receivers in RS-422/TIA-422 and RS-485/TIA-485 networks respond only to differential voltages and not common-mode voltages, the induced noise is ignored.

RS-422/TIA-422 is a simplex (one-way) communication standard, while RS-485/TIA-485 is a duplex (two-way) communication standard. Both support more than two devices on a network segment. With RS-422/TIA-422, it means one transmitter and multiple receivers while with RS-485/TIA-485, this may include transceivers i.e. devices able to transmit and receive data at different times (half-duplex). Four wires are required to connect two such devices when full-duplex (i.e. when simultaneous two-way communication takes place). A full-duplex is only practical between two devices.

RS-422/TIA-422 and RS-485/TIA-485 specify positive and negative voltage differences (measured between each dedicated wire pair) for its signaling both for the transmitting and receiving devices. A receiver must recognize any signal that is more negative than -200 millivolts as a ‘’mark’’ (1) and any signal that is more positive than +200 millivolts as a ‘’space’’ (0). You will notice that these voltage thresholds are much lower than those specified for RS-232/TIA-232 standard (±3 volts) because of the lack of noise on differential signal lines compared to ground-referenced signal lines.

The maximum cable length of RS-422/TIA-422 and RS-485/TIA-485 networks is 1200 meters with the long cable lengths and higher data rates made possible by differential signaling, some applications may need termination resistors to eliminate reflected signals.

Related: What is Fieldbus in Industrial Communication Networks?

There is no established pin numbers or labels (standard) for the differential transmit and receive conductors as the case with RS-232. Usually labels A and B, alternative labelled – and + or A– and B+ a re used. For example RS-485/TIA-485 full duplex connection is illustrated in figure 1(e) below:

Accidentally connecting the wires incorrectly in RS-422 or 485 will not damage the electronics. You can observe the ground conductor in the connection in figure 1(e) even through the differential signaling doesn’t require one. The ground connection in this case is employed to ensure that common-mode voltage does not become excessive since the local circuits will not work properly when exposed to significant common-mode noise.

With the connection in figure (f)  above, two way communication can occur over one pair of wires in a RS-485 (TIA-485) network.