i have a Modbus RTU system with MicroLogix 1400 as master and 3 Eurotherm instruments (2404, 2704)+ Allen Bradley drives as slaves (total 7). Facing problem of communication going too slow if one or more slaves are switched off or goes fault. sometimes communication stops also. in normal case when all are healthy then communication is very good.
used channel 0 and NC01 cable.
Slow communication throughput is the nature of 'sequential polling' Modbus when a slave fails to respond for any reason because the master/client waits for a response during the entire 'time-out' period before moving onto the next poll in the sequence.
You can shorten the time-out period or do some coding to eliminate those slaves that are Off or faulted from the polling schedule.