Invertek Optidrive Modbus Communication issues using Minimal Modbus on and RPI3

[masterpo88]

I'm am controlling an OPTIdrive e from an RPI3 running minimal modbus

I can read and write the all RTU registers except RTU-1, the control word, correctly by subtracting 1 from the Optidrive RTU register address.  When I write RTU-1 the Optidrive returns an error message the the data is illegal.  However,  when I read RTU-1 without error  the value has changed to what was written but the corresponding control function did not happen, e.g., run to stop or stop to run. 

01 06 00 00 00 01 48 0A   returns 01 86 03 02 61               Write 00 01 to the command word, illegal data error returned

01 03 00 00 00 01 84 0A   returns 01 03 02 00 01 79 84      Read command word returns 00 01

01 06 00 00 00 00 89 CA   returns 01 86 03 02 61               Write 00 00 to the command word, illegal data error returned

01 03 00 00 00 01 84 0A   returns 01 03 02 00 00 B844     Read command word returns 00 00

Any ideas?  For some reason the RTU-1 control word does not work from the command but all the other register seem to function properly.  

[K C]

In one of the manuals I read using register 44 and 45 for read and write access.

Is this also applicable for your device ?

Kees