I have a working system which uses a 3rd party Modbus master talking to a CP341 Modbus slave. All functions work fine until the master tries to read a block of registers starting at 511. This results in an 'invalid register' type error from the CP341. If I try and read the same registers, but starting at 512, everything is fine. Is there a restriction on 'odd-number' registers?

 

It may require you to handle register-pairs. This is common for floating point data and long integer data where the data is 32 bits long and spans across the 16 bit registers. I do the same in my products.

Max