null modem cable


Thread Starter


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.

What you want is a standard null modem cable. They can be bought in any electronics store. There will be basically 2 kinds of 9-25 pin cables available, MODEM cables, and NULL MODEM cables. Just make sure you get one labeled NULL MODEM.

They will have pin 2 connected to pin 2 and pin 3 connected to pin 3 (thats TD and RD crossed). Pin 5 on the 9 pin will be connected to pin 7 on the 25 pin (thats signal ground). Pin 4 on the 25 pin will be connected to pin 7 on the 9 pin, and pin 5 on the 25 pin side will be connected to pin 8 on the 9 pin (thats crossing RTS and CTS). If you do not want to use RTS and CTS, they need to be jumpered together on each side. There are null modem adapters that can do this for you, if you want to avoid making your own cable. And Finally, i cannot see why you only want 1 side of the TX/RX pair connected, but if you are hell bent on it, then Pins 3 and 3 are the important ones to you. 3 is transmit on the 9 pin and recieve on the 25 pin.

Here's a website that might be able to explain it better if you had trouble following along. They have pictures n stuff.

i think thier null modem pinout is confusing. If you look at it, just keep in mind that to get a null modem cable, you have to go from 1 side of the X to the other side. The way they list both sets of pin numbers makes the whole thing look like a straight through cable to me.


Lynn at Alist

I see you are repeating yourself - no one answered the first post because you didn't do your homework and provide what pins are what.

The pin-out of the PC is well documented.

The pin-out of the "controller(device)" could be anything one can imagine. Find the manual for the "controller(device)" and see what they suggest. It may NOT even be RS-232 - it could be current loop. If it is RS-232, then likely it's either a DTE or DCE.

If they don't say (or more likely you don't have one), then you'll need to use a multi-meter to determine which signals are drivers (should be forced to -6 to -12 v when idle) and which are receivers (they should float around 0v or be within +/- 1v of 0v).

Best of Luck - LynnL
what happens if I don't connect RTS and CTS together? I mean is it at all possible to make the communication without this connection?


Lynn at Alist

Usually it does. However I've seen highly-paid engineers spend literally days trying to trouble shoot a data comm problem that turns out to be the lack of a DTR/DSR or RTS/CTS loop-back (or connection). Worse, some products/apps change their expectations of DTR/DSR or RTS/CTS as new revisions come out without warning users. So what worked yesterday may not work tomorrow.

Yes, you can create a pure 3-wire-only null-modem cable. yes, it will work for a most of your present/future systems. But yes, there will be a FEW it won't work for.

Since adding the DTR/DSR and CTS/RTS loopback takes you 5 minutes or less when you're in a cable-making mood, I always say add them to ALL cables and save yourself the headache of which systems care and which don't.

best regards - LynnL

Abdul Khaliq

Pin Pin name Pin Function Input/Output
1 DCD . Data carrier Detect. NotConnected
2 RX . Receive Data. Input
3 TX . Transmit Data. Out put
4 DTR. Data Terminal Ready. Out put
5 GND. Signal Ground. Bi-dirctional
6 DSR. Data Set Ready. Not Conncted
7 RTS. Request to Send. Out put
8 CTS. Clear to send. Input
9 RI. Ring Indicator.
(1)Requested above SCP ( Serial communication Port ) 9 pin fimail adupter cable connection to 25 pin fimail connection for serial port EPSON 300 printer.
(2)ICP (Isolated Communication port )requires 9 pin fimail adupter cable connection to 25 pin fimail straight-through cable connection for serial port EPSON 300 printer.

Many thanks
Best Regards
Abdul Khaliq
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