there's a util called commlite which will monitor serial comms from a PC with no hardware, i.e. it intercepts the OS comms calls & logs the data. It can also be used to monitor external data via the com port.
Note that using USB you will not get good timing info as serial data gets packetised over USB. This means if monitoring 2 lines with 2 USB adaptors you may not always be able to determine the relative order of the messages.
What kind of serial connection is on this line ? A true full duplex communication or a simple half duplex protocol ?
If this is a simple half-fuplex protocol, these can be analyzed with a single port and two diodes.
Connect the cathode terminals of the two diodes to the Rx pin on the monitor device.
Connect the signal ground of the monitor device to the pass-through line signal ground
Connect the anode of one diode to the Rx signal of the pass-through line
Connect the anode of the other diode to the Tx signal of the pass-throuh line
Start a serial line monitoring program listening to the monitor device Rx pin
The bytes on the half duplex line will be on the same line on the monitoring device, but in a typical half-duplex protocol it is not hard to determine, which byte belongs to the pass-through line Tx or Rx signal.
Even if the monitoring device is an Ethernet/serial or USB/serial converter with huge latencies, the Rx and Tx characters are in correct order.
When using two separate USB or Ethernet converters, the Rx and Tx bytes might appear in wrong order due to different latencies.
Of course, this single port with two diodes connection does not work if there is simultaneous (full-duplex) traffic in both directions, since any overlapping byte would be garbled and possibly all the following bytes might also be garbled due to loss of synch, until there is at least a single character time long silent period with no activity on neither Rx nor Tx line. This full duplex garbling only affects the monitor line, the pass-through traffic would continue normally.
If you can solve the lack of serial ports problem and have a PC with two serial ports supported by DOS then I'd recommend 'dlm'; see my reply in this group to a similar request a short time ago. Search this group for 'dlm dos pc'.
They sell a tool by the name of "Docklight" which not only monitors 2 serial ports but does a huge number of things more for serial debugging. And it works with USB->SERIAL converters. You may download a free no-timelimit evaluation version. This tool has proven to be top value for all our serial problems. No fears: While it is a German company, everything is in English.
Regards Ulrich
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