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I have been reading the different discussions and I realize that a slave device cannot communicate an alarm condition to the master. The master has to poll the slave.

What I want to do is to program an alarm condition to my slave device that when the condition is met the slave would set a bit in the alarm register but would also set a DIO to high without the interaction of the master. The slave would do this all on its own. I want to know if this is a legal operation in Modbus protocol.

Thanks.

Some slave devices do automatically turn outputs on or off according to whether an input is within set limits. That has nothing to do with the Modbus protocol however. Modbus is a communications protocol. It is all about what bits go out over the communications cable. It has nothing to do with whatever else the slave or master can do in their spare time. You will need to go over the data sheets and manuals for various manufacturers to find an I/O module that does what you want.

Why would that not be legal?

Just because it's a slave doesn't mean it can't do stand alone control, slave refers to the communication only not operation.

I assume you are going to hard-wire the DIO back to the master to let it know there's a change.

Roy