Thread Starter

Monroe Strike

I have a MODBUS MB-3100 PROSOFT module to communicate with an Allen Bradley PLC5-80e on the same chassis, and I would like to know if the
scanner clears the data area when the connection to a particular Modbus slave device is lost.

If the scanner does not clear the receiving data area then is there a specific control bit that I need to send to the Modbus module to clear the data area or do I have to create code to send zero's explicitly.

Monroe Strike
FLS Automation Denmark.

Yosef Feigenbaum

The module will not clear the rxed data area. You will get error flags on the commands that the module was unable to execute. These error flags arrive in the upper 12 words of every BTR that you get from the module. It is important that you latch these bits if you want to use them in your program since they will be reset by the module at the onset of every tx (which includes retries) of the message in question so what you will see is a toggling of these bits.

You will be able to clear the data area in the
module by writing to it but it seems to me that
you'd be better off using the error flags and
clearing the data area in the PLC will a File Fill instruction.

(8{)} ( .)

Michael Trask

You could use the Master error table to tell if any individual command in the list has failed. However the data remains a problem. It is not possible to clear the memory in the module but you could choose not to copy that block of data into the plc based on the error code of the command.
As an example slave 1 results may be contained in
Block ID 0. If the commands for this slave != 0 then you could disable the copy statement on the BTR rung.