I ran into this issue because I don’t really have any old hardware or components, but I do have a bunch of new stuff. Like, every HP switch comes with a console cable. Resulting in tons and tons of console cables.
The issue was that I needed a null modem cable, but only had the more modern console cables with an RJ45 connector on the other end. The cables I am using are HP 5184-6719 Console cables.
- I am under the impression that if I simply had a standard RJ45 coupler, I could connect them that way and this would work. But I don’t have any of those and needed a null modem cable with the quickness.
HP 5184-6719 Console cable
“Rainbow” wiring layout.
This is what the end looks like, in case you were wondering if you have the same one.
I personally tested these cables with a continuity test and found to which pin each wire was associated with, and then associated those with the proper directives in RS232 standard. Refer to the pinout chart below.
|Clear to send||pin 8||black||CTS->RTS|
|Request to send||pin 7||purple||RTS->CTS|
|Ground||pin 5||orange / yellow|
|TX ( Transmit data )||pin 3||green||TX -> RX|
|RX ( Receive data )||pin 2||red||RX -> TX|
Since you can read the pinout and don’t have to test yourself now, feel free to snip+strip your cables and get twisting!
RS232 9-pin Null Modem Handshake Diagram
According to the chart below, the standard crossover for a handshaking null modem cable is to cross-connect the TX/RX and the CTS/RTS pins for both ends. I decided I would cross over the grounds in this case, too. (orange to yellow)
Make it look as nice or as DIY as you like, so long as the connections are proper.
To make it simple:
(do this for each serial cable)
Connect green to red
Connect black to purple
Connect yellow to orange
- Again, you will do this for both serial cables, so you will end up doing this twice. This is how you create the crossover.
I opted to do it with what I had on hand (i also didn’t have a simple wire-cutter tool so I used the blades on my RJ45 crimper), but you can make it nicer with different techniques.
- CAT6 coupler
- Small wire-caps
- Electrical tape
Now you have a working null-modem cable and you can do things on your old equipment with no modern serial port!
I hope this helped you accomplish whatever you needed to do with your null modem cable.