Hi,

if I good remember (also can say only from older system cfg) Slave address must be set correctly as mandatory but no any pending data exchange is expected at change cfg time. Correct register address and then number of next amount of registers to read, here is expected with same structure. In case of not same structure, Modbus is very restrictive here so try uncut questions to Slave to more prepared questions, each with specific Slave address, start of register address and amount of registers to read. Practically, MB RTU is half duplex so you can poll only one data at time, wait for answer or error, diagnose this and stop block execution. Change next parameters for other Slave and similar procedure for next one. No problem with other help here