Modbus RTU Emulator


Thread Starter

Geoff Levings

I am looking for a package that will allow a NT or Win98 system to display and send Modbus packets. I need to be able to emulate the registers in a Modicon PLC (984 or Quantum). Is anyone able to suggest a solution - prefer freebie
L has both a "Master" simulator (modscan32 - allows you to poll slaves) and a "Slave" simulator (modsim32 - allows you to simulate one or more slaves). They operate in a shareware mode which limits operation to a few minutes.


Lynn August Linse, Senior Product Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected]
Tel: (949)450-7272 Fax: (949)453-7132
Try to use Modsim32 & Modscan32, one for server & one for client both provide the ASCII,RTU & Modbus/TCP mode. Web site: . Not freebie but unregister version
can play with a couple of minutes before the service were shutdown.

Al Pawlowski, PE

Green Leaf System's Harmony SCADA package for Macintosh had (in 1996) that function built in as a troubleshooting and configuration aid. What
the package does is allow you to copy your system/RTU configuration and then set two (software) switches that reverse the AI/AO's and DI/DO's and turn the configured comm link into "responder" mode. The copied configuration then can respond to the original configuration and
simulate all, or part, of the RTU's using whatever comm protocol the link/s were set for. To feed values to your Central, you just type
values into the "copied" points as if they were setpoints. Multiple instances of the program can be running at the same time so that you could have the original polling the copied/reversed setup through the machine's actual serial ports and attached comm's equipment as desired. You can also just have them talking internally since Harmony's comm links are logical and both copies could be attached to the same physical port.

Between the exceptional comm's monitoring screens and this "responder" mode you had a very neat troubleshooting and configuration tool. I used
it a number of times in the shop and in the field.

Would be a nice function for the big Wintel SCADA suppliers to add to their packages.

Harmony was a solid, advanced product, but I don't know whether it's still available. Green Leaf was in Berkeley, CA.
if you are looking for this for testing purposes, I know of an OPC server for Windows 95/98/NT that functions as a modbus slave. it comes with a
test client you can run on the same PC to stuff values into the registers. As for freebie, it runs for 2 hours at a time in demo mode -- download it at


John Weber
Software Engineer
Qualitrol International