Delta RMC151 - Manual (Page 559)

6 Communication
The RMC75S serial port 1 supports 2-wire RS-485. It does not support 4-wire RS-485.
RS-485 uses the 8-pin connector block on the RMC75S CPU module (pins 6, 7 and 8 are
for power to the controller). The pin-out is as follows:
Pin RMC70 Label RS-485 (2-wire) Function
1 +T/R Rx/Tx B (+)
2 Trm Jpr See RS-485 Termination and Biasing
3 -T/R Rx/Tx A (-)
4 SCmn Common
5 Bias Jumper See RS-485 Termination and Biasing
NOTE:
Pins 2 and 4 are for termination and biasing. See the RS-485 Termination and Biasing topic for
details.
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 +.
Cabling
All cabling for balanced or differential communications should consist of twisted pairs.
Because the RMC's RS-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 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. See RS-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 100Ω 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 100Ω 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-485 depends on the baud rate and termination. At
higher baud rates, termination allows longer cable lengths. For details on the effects of
termination and how to apply it, see RS-485 Termination and Biasing.
The following chart shows the maximum cable length for RS-485 with and without
termination:
Maximum RS-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
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