RJ25 to DB9 serial port adapter

Hopefully this post will help others in a similar predicament. Recently I needed an APC 990-0144 RJ25 to DB9 serial cable so I could configure one of their Switched Rack PDUs but (rather predictably) the original wire was nowhere to be found. Extensive Googling did not provide many answers so I set about with a multi-meter to make my own. To make a long story short...

The pinouts of the RJ25 socket/plug, the RJ45 plug and the RJ45 socket to DB9F adapter are shown in the following table. The minimum three wires to provide the terminal connection are highlighted in bold. (Note that most of the Cisco adapters only have these three wires connected.)

RJ25	My cable	RJ45	DB9F
1 - Not connected	Blue
2 - GND	Yellow	5 - GND	5 - GND
3 - Rx	Green	2 - RX	2 - RX
4 - Tx	Red	3 - Tx	3 - Tx
5 - CTS	Black		8 - CTS
6 - RTS	White		7 - RTS

I took a trip to Maplin and picked up a "FCC68 6P6C" (RJ25) IDC plug, one metre of "FCC68 6-core" cable. From my own box of tricks I collected an RJ45 IDC plug and a spare Cisco RJ45 to DB9F adapter (so as to eliminate the need for any soldering). Assembling the cable as per the table above, and shown close-up in the picture, worked first time.

As only pins 2,3 and 4 from the RJ25 are used, if you have a more common and spare RJ11 (6P4C) telephone cable you could chop one end off and terminate it with a RJ45 in a similar way to end up with the same cable.

This cable would appear to have the same pinout as used on, amongst others, some old DEC equipment and IBM FastT200 controllers (the IBM part number for the same cable appears to be FRU 19K1179).