MODBUS use with Motor Drives

Using Interfaces with motor drives gives setup personnel an easy way to change drive parameters. This usually happens when machines are required to run several different jobs. Their configuration, tooling and drive parameters need changing. Many motor drives use MODBUS protocols for communications to a PC. The PC is used to configure the drive before installation. Once installed, configuration changes can be made from a laptop or something more dedicated like a handheld device provided by the drive manufacturer or an HMI with appropriate protocol capabilites.

You would think this would be an easy HMI project. But you might want to check some things out. Be sure you can get the proper documentation; such as a list of drive parameters and their equivalent MODBUS register assignments. Sometimes the MODBUS assignments are in Hexadecimal, other times they are in Decimal notation. Be sure you are using the correct addresses, converting hexadecimal to decimal as needed. Another thing to check is the starting address of the MODBUS registers. Some drives start with register 400000, others start with 400001. This one word offset can throw things off and strange values appear on the HMI. Also watch out for the register order of double words or floating point values. Again, if drive data seems out of place or strange behavior is happening check if the data is low order or high order sequenced.

I had a call from a developer designing a system using Kollmorgen Drives. This drive communicates MODBUS TCP/IP. Some values were not appearing as they should and the customer and I had a tough time figuring things out. Eventually, the problems were resolved. The drive registers started at word 400000 and the register word order was reversed (high order sequence). Once this was realized, project development proceeded normally.