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:
None of the two partners is running Windows, or the system is closed for intrusion (i.e. no administrator access)
The program in question doesn't use the Windows API for accessing the serial port
(I have seen one, really!)
Full duplex with status lines
This one allows spying the status lines. It is derived from the Other Cables above.
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:
There is no Ring Indicator (RI) as the computer doesn't wake the modem.
There is no Data Carrier Detect (DCD) as there is no modulation on computer at all.
The two status lines are crossed as usual.
How to use the cable
This may be a useful setup for the spying computer:
Ensure you have a computer equipped with two serial ports.
USB↔Serial converters will work fine but status line toggles are not seen when shorter than 1 ms.
Start two instances of any telnet application, e.g. putty or HyperTerminal, one for each spy port.
Guess the baud rate, and the number of data bits. Set stop bits to 1. Disable handshaking.
In rare cases, baud rates can differ in both directions!
For a log file with interleaved serial protocol data in timed order, run PortMon for the two ports.
Use hexadecimal logging when binary data is transferred.