Changing the Modbus slave address at runtime (I think that's what you mean) would be very tricked out. That sort of feature would be in Control Microsystems' wheelhouse, not Allen Bradley's
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I'm guessing you want to write one and the same program to many sites and have each program self-identify on startup. OK, but how? With an external thumbwheel or equal? It'scertainly a valid problem, but I don't a solution off hand.