Null modem cable

Just the other day I brought this old RS232 null modem cable from home, as I needed to hook up the oscilloscope to the PC for some waveform capture.

I plugged it in and it wasn’t working, so I spent some time troubleshooting. I used a multimeter to check the continuity on the RX and TX pins, and when that failed I assumed the wire must be broken inside. Somehow I already went ahead to cut a ring around the gray wire sheath when I opened up the connector shell.

After looking at the wiring, it turns out the cable was of some odd configuration. One end was connected to the RX and TX pins, but the other end was connected to some other pins (which were the RS232 control lines). I ended up re-soldering the RX and TX pins on one end of the cable, which made it a real null modem cable.

But this was not until after I cut off the head and unsheathed the wire, since it was already done halfway.

Next time, I will remember to check the wiring first before assuming the wire was broken.