Listening to Serial Com Ports that are In use

Question

I'm dealing with some legacy systems that are using RS232 to communicate with peripherals. I'm not very experienced with COM interfacing. I have some code that can open and use COM ports, but it can't open ports that are used by other applications. I need to black box the packets so that we can use the same protocol for updated communications.

Is there any way to "middle man" incoming packets to an open COM port and detect what packets are being sent? I'm using .NET, but I'm open to any type of solution.

(I found this out there, but I don't think this will work for me.)

Answer 1

I've used com0com - it's great for setting up virtual com ports - which doesn't help you at all.

The COM port interface is basically a 'file read'. My application throws an exception when I try to connect to a COM port that already has another instance reading from it. I'm not sure if you could try opening it as a 'read only' instead of read-write, but it's worth a try.

You should be able to write a virtual com port that can fork off your data to a log file. Com0com is open-source, so you could use that as a starting point.

Another possible solution could be to pick up an rs232 splitter cable forks the serial signal to another serial port.

Or yet another possibility is a Serial Sniffer program (or an open source sniffer).

Or try the hub4com app from the same com0com website!

Answer 2

I have been down this same path. A hardware splitter is the easiest solution.

hub4com setup will involve the "Add New Hardware" wizard. If you have a lot of machines, geographically separated machines, or users who are not technically savvy and lack the permissions necessary, installation could be awkward.

If this is a legacy application, does it run in ntvdm? If so, you could run it in DosBox instead and alter the DosBox code to write to a file in addition to sending/receiving to/from the serial port. DosBox is cross platform as well.