Delta RMC101 - Manual (Page 495)

Serial (RS-232/422/485) 5.5
Communications 5-245
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)