Creating A Virtual RS-485 Null Modem

Question

I'm in the processes of developing a RaspPi program that will be responsible for data-logging sensors on an RS-485 based Modbus network.

Development is being performed on a virtual machine running Linux on top of my windows laptop. Currently the two are linked via a virtual null COM modem that I set up with com0com on the windows side of things.

From the debug information on my applications end I know that the program is outputting a valid MODBUS RTU frame. However when my slave simulator (Unslave) receives the information it appears to have been corrupted with the values changed and the frame no longer valid.

Is COM0COM comparable with this kind of use (was unable to verify via google)? Is there a better application/framework to use for this case?

Any help would be greatly appreciated. There does appear to be a commercial solution that advertises RS-485 support but ideally I would like to utilize a more open solution.

Answer 1

Since you are already developing on Linux, one option may be to forget about Windows and use the equivalent Linux tools and test everything inside the VM.

• For the null-modem cable simulation, the tty0tty kernel module. (You'll probably have to compile it yourself, but it's all quite standard as these things go on Linux.)
• For a slightly easier but less realistic simulation there is socat. It can do forwarding over network, but some applications don't work with it (because of specifics of the IOCTL they do).
• As a modbus client (master) for quick testing is mbpoll, and there are probably others.
• You could use pymodbus for testing / simulating devices, which can do both server (slave) or client (master).

Another option is to use a real null modem cable. (A hardware solution to side-step complex software setup.) For example you could simply plug in two USB dongles (probably RS232 instead of RS485) and map one of the USB devices into the Linux VM (so it uses the Linux driver).

But in the end, it really depends what you want to test. (You haven't really told us.)

• If you just want to test the logic of your program, maybe abstract the serial interface away and write unit-tests. Or maybe your application can support both Modbus-RTU and Modbus-TCP, so you can do most tests over TCP, side-stepping all serial-port issues.
• If you want to test RS485 specifics, better use a real RS485 bus (hardware). There are some specific timing issues (like some devices not giving up the bus fast enough, destroying data from other devices) or IOCTL setup on the Linux side that you can't reasonably simulate.