Communication between Siemens HMIs and non-Siemens controllers

Ever wanted to use those very nice looking Siemens touchpad HMIs with a third party controller Siemens doesn't officially support? On a recent project I was presented with the following scenario: a customer already had several Siemens touch-screen HMIs in stock, and wanted to use one to help operators control an Elau motion controller. Of course to do this I had to get the HMI and controller to talk to one another. The HMI understood about 20 different protocols — everything from Allen Bradley DF1 all the way to Telemecanique's Uni-Teleway. But every single one was proprietary, and Elau's wasn't on the list.

After a little more looking around, however, I noticed two very familiar names: Modicon Modbus and Modicon Modbus TCP/IP. As most of us in the systems integration business are aware, Modbus is one of the most widely used protocols in the industrial world. And it is completely open and standardized. We were in the clear — or so I thought.

Just to be cautious, I decided to ask Siemens tech support about their Modbus protocol support. To my surprise, two separate representatives told me that their version of Modbus was different than the standard, and there was "no way" I could make it work with anything but a Siemens PLC. Maybe I am just stubborn, but that didn't make sense to me. I thought Modbus was Modbus, wherever you go. I found pinouts for the HMI and controller RS 232 ports, and set about making a cable. To my chagrin they were right. I could write to the controller, but not read from it.

At my last straw, I plugged in an Ethernet cable between the two devices and gave Modbus TCP/IP a shot. Boom — barely any setup needed and I was up and running. I can only guess that Siemens uses some slight twist with their serial Modbus protocol that makes it non-standard, whereas the Ethernet's more robust, native error checking makes that unnecessary. Whatever the case, this is good news — despite Siemens tech support's claims, their HMIs will work with any controller that has Modbus TCP/IP support (or even a device that just has Modbus serial, if you just grab one of these easy-to-use devices). Good to know, so I thought I would pass on the knowledge!