RS-232 Wiring for the RMC75

This topic describes the wiring of the RS-232 serial port on the RMC75S. For details on wiring the RS-232 Monitor Port, see the Monitor Port topic.

Connectors

Both of the 9-pin male DB connectors on the RMC7xS are used for RS-232 communications. A 9-pin female connector is required on the RMC75 end of the connecting cable. The other end of the cable must match the master/host hardware, which may be a 9-pin, 25-pin, RJ-11, connector block, or other connector.

Pin-Out

Pin assignments on the RS-232 9-pin connector:

Pin #	RS-232 Function
1	DCD- Not used by RMC
2	RxD - Receive Data
3	TxD - Transmit Data
4	DTR - Not used by RMC
5	GND - Common
6	DSR - Not used by RMC
7	RTS - Ready to Send
8	CTS - Clear to Send
9	Not Connected

Note:
The RTS and CTS pins need only be connected if the RMC75S serial port has been configured for Hardware Flow Control.

Cabling

A null-modem or crossover cable is typically used for RS-232 communications. The RMC75 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.