Delta RMC151 - Manual (Page 558)

RMC70/150 and RMCTools User Manual
A null-modem or crossover cable is typically used for RS-232 communications. The
RMC70 RS-232 communications require only three conductors in the cable: RxD, TxD,
and GND. See the following wiring diagram for details.
Delta recommends that a shielded cable be used to limit susceptibility to outside electrical
interference.
Cable Wiring
Note:
The RTS and CTS wires need only be connected if the RMC75S serial port has been configured
for Hardware Flow Control.
Cable Length
One of the characteristics that limit the length of an RS-232 cable is capacitance. Most
cables have a capacitance rating in pF/ft. Use the following formula to calculate the
maximum distance signals can be reliably transmitted for a given cable capacitance.
MaxLength = 2400 pF / ( C + ( Shield * C ) ),
where
Shield = 2 for shielded cable and 0.5 for unshielded cable
C = capacitance rating of the cable in pF/ft
2400 pF = derived by taking the maximum capacitance specified by ANSI/TIA/EIA-
232-F (2500 pF) and subtracting 100 pF for the input capacitance of
the receiver
Example:
The cable that is being used is shielded and has a capacitance of 15 pF/ft.
MaxLength = 2400 pF / ( 15 pF/ft + (2 * 15 pF/ft) )
= 2400 pF / ( 45 pF/ft )
= 53.3 ft
Note:
If you need to run your communications farther than allowed by this formula, then you must
either use a cable with lower capacitance or use RS-485.
6.10.3.5. RS-485 Wiring for the RMC75S
Connectors
538 Delta Computer Systems, Inc.