null modem


Thread Starter

hossein tavana

Hi, I need to make a null modem cable for the following situation.can anybody please tell me which pins(TD,RD,...) should be connected together for this specific case and how:
-Situation:I want the data which are put on the 9-pin COM port of a PC to be delivered to the 25-pin RS-232C serial port of a controller(device) regularly.No data is necessary to be sent back from the controller to the PC.
-How the cabling will differ if I want data to be sent back to the PC?
-Since the controller can work in both the full duplex mode and the half-duplex mode,and the PC should be in the similar mode,does a PC work in ?
Pin-out for 9pin: RD=2, SD=3, DTR=4, SG=5, DSR=6, RTS=7, CTS=8

Pin-out for 25pin: SD=2, RD=3, RTS=4, CTS=5, DSR=6, SG=7, DTR=20

Null-modem pin-out is pin-to-pin, i.e. 1-1, 2-2, 3-3, 4-4...

A communication cable for RS232 is inverted, i.e. RD-SD, SD-RD, SG-SG, TDR-DSR, DSR-DTR, RTS-CTS, CTS-RTS. Depend of application, the DSR can be jumped with DTR in the connector. Samething for RTS-CTS.



Daniel Chartier

Hello Hossein;
Try thje following link to B&B Electronics webpage:

The cable you are looking for is designated as a MODEM cable.
Follow the links to their support page for articles and whitepapers on connectivity. They should be enlightening.
By the way, use the same cable connections wheteher you plan to sebd data both ways or only one way.

Hope this helps,
Daniel Chartier
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