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Monday, November 19, 2018
The Motion Control Community Forum, sponsored by Control Technology Corporation.
Rexroth FE Modbus Control
Problems controlling a Rexroth FE variable frequency drive over RS485 MODBUS

I've been experiencing a very strange issue with a Rexroth FE variable frequency drive. When i control or configure it with the original software it works fine. But when i try to read or write the holding registers with a master emulator, after the register map in the manual. It answers nothing and/or has no effect in drive. All this in spite of having set the remote mode in config.

If the configuration software didn't work, it may be some configuration or communication problem. But taken that it works, the fact that it doesn't answer MODBUS commands is driving me crazy.

Do you have some kind of explanation for that? Has anybody faced the same problem?

1. Does the original software" use the same RS-232 or RS-485 port as you're using for Modbus?

On a lot of devices, the mode that the port operates in has to be dedicated to either 'configuration' or 'communications' and it won't switch back and forth between those modes. The serial port functions in one mode or the other. Are you sure the 'remote' mode converts the serial port into a Modbus comm port?

2. Some generic serial Modbus checks

a. Are you using the same hardware with the master emulator as you are with the configuration software - 232/485 converter, whatever ?

Changing serial hardware can lead to the problem where the 485 lines are backwards and need to be swapped, or serial setup defaults to different settings. Be aware that many devices do not recognize serial setting changes until the power is cycled on/off.

b. The master could be reading or writing from/to the wrong register address, offset by one, where 00020 is really 00019. Depends on whether the address/registers are numbered as one based or zero based.