RS-422/485 Wiring for the RMC SERIAL

Connectors

Both RS-422 and RS-485 use the 6-pin connector block on the RMC SERIAL module. The pin-out is the same for RS-422 as 4-wire RS-485:

RS-422/485 (4-wire) Pin-out

Pin	RS-422/485 (4-wire) Function
1	Tx A (-)
2	Tx B (+)
3	Rx A (-)
4	Rx B (+)
5	Common
6	Case

Two-wire RS-485 uses the following pin-out:

RS-485 (2-wire) Pin-out

Pin	RS-485 (2-wire) Function
1	Unused
2	Unused
3	Rx/Tx A (-)
4	Rx/Tx B (+)
5	Common
6	Case

Note: Some manufacturers use A and B labeling, while others use + and - labeling. If you need to interface to equipment that uses an alternate labeling scheme, keep in mind that A corresponds to - and B corresponds to +.

See also General Wiring Information.

Cabling

All cabling for balanced or differential communications should consist of twisted pairs. Because the RMC's RS-422/485 interface is isolated, the signal common must be run alongside or in the cable. Therefore, for a two-wire network the cable must be either a one-pair cable with a separate ground line that is run externally or a two-pair cable in which one pair is used as the common. For a four-wire network this requirement changes to two-pair and three-pair. For a clean cabling solution, Delta recommends the option using an additional wire pair.

Another consideration when selecting communication cabling is the impedance of the cable. This impedance should match the termination resistance that is used. The RMC SERIAL module has a software selectable internal 120W termination. See RS-422/485 Termination and Biasing to determine whether or not your network will require termination.

No cable characteristics are specified in the TIA/EIA-422-B and TIA/EIA-485-A standards, but the RS-422-B standard does recommend 24AWG twisted pair cable with capacitance of 16 pF/ft and 100W characteristic impedance. These specifications will work well for RS-485 as well. One good choice would be to use Category 5 Ethernet cable. Category 5 Ethernet cable has a capacitance of 17 pF/ft max with 100W characteristic impedance. It is commonly available as shielded twisted pair (STP) or unshielded twisted pair (UTP). If this is not suitable then there are a number of manufacturers of communications cable such as Alpha and Beldon Wire and Cable.

Cable Length

The maximum cable length for RS-422 and RS-485 depends on the baud rate and termination. At higher baud rates, termination allows longer able lengths. For details on the effects of termination and how to apply it, see RS-422/485 Termination and Biasing.

The following chart shows the maximum cable length for RS-422 and RS-485 with and without termination:

Maximum RS-422/485 Cable Length:

Baud Rate	Max Unterminated Cable Length (ft)	Max Terminated Cable Length (ft)
115,200	475	3250
57,600	950	4000
38,400	1900	4000
19,200	3750	4000
9,600	4000	4000
4,800	4000	4000
2,400	4000	4000