Build an Ethernet and RS-232 tap using the 5-in-1 cable

The Five-in-One Admin’s cable is a very handy cable. It was featured in the premiere edition of Make. The five-in-one cable is an Ethernet cable, a crossover Ethernet cable, a modem cable, a null modem cable, a serial/Ethernet loop-back cable, and a Cisco console cable. That’s a lot of functionally in just one cable. The elegant thing about this cable is that its components are dual use. For example, the cross-over cable doubles as a null-modem cable.

Recently, I’ve been working on projects that require an Ethernet tap and an RS232 tap. A tap is an in-line device which allows one to passively sniff a connection. Taps can be used to debug problems which would other wise be tricky to debug. Taps also have applications in security monitoring systems like Snort. I could have bought a RS232 tap and an ethernet tap, but after looking over the circuit diagrams I noticed something. The circuit diagrams for the Passive Ethernet Tap and the Full duplex RS232 sniffer cable are very similar. In fact the only difference between the two cables is that the handshake signals on the RS232 sniffer cable have been looped back.

So in the spirit of the 5-in-1 cable, I built a system of connectors to allow me to tap both Ethernet and serial communications (not at the same time, of course).

The construction wasn’t too hard. Basically I just built the five-in-one cable described here. Then built the Passive Ethernet Tap described here. The only thing left to do was to build two of my own custom loopback cables.

Here’s instruction to make your own