Serial Spy Cable

See also: Other Cables

When to use

You need one to spy data traffic between two devices, and you cannot use the nice PortMon (or similar) sniffing tool on one of the devices. This is the case when:

Full duplex with status lines

This one allows spying the status lines. It is derived from the Other Cables above.
schematic (b&w)
Full duplex RS232 spy / monitor / sniffer cable
Note that the taps are somehow asymmetric by design. The Spy port 1 gets the data and status sent in A direction, from device to computer. This will include the rarely used Ring Indicator at pin 9.

The Spy port 2 gets the data and status sent in B direction, from computer to device. There are only two status lines:

The two status lines are crossed as usual.

How to use the cable

This may be a useful setup for the spying computer: