Communications
6.2 PtP, USS, and Modbus communication protocols
Easy Book
80 Manual, 05/2009, A5E02486774-01
6.2.3 Library of Modbus instructions
The MB_COMM_LOAD instruction configures a port on the
CM module for Modbus RTU protocol communications.
You can use either the RS232 or the RS485 CM module.
Your user program must execute the MB_COMM_LOAD to
configure a port before either a MB_SLAVE or a MB_MASTER
instruction can communicate with that port.
The MB_MASTER instruction allows your user program to
communicate as a Modbus master. You can access data in
one or more Modbus slave devices.
An instance data block is assigned when you place the
MB_MASTER instruction in your program. Use this DB to
specify the MB_DB parameter on the MB_COMM_LOAD
instruction.
Execute all MB_MASTER instructions for a given port from the
same OB (or OB priority level).
The MB_SLAVE instruction allows your user program to
communicate as a Modbus slave. A Modbus RTU master can
issue a request and then your program responds via
MB_SLAVE execution.
You assign a unique instance DB when you place the
MB_SLAVE instruction in your user program. You use the DB
name when you specify the MB_DB parameter on the
MB_COMM_LOAD instruction.
Execute all MB_SLAVE instructions from a cyclic interrupt OB.
The Modbus instructions do not use communication interrupt events to control the
communication process. Your program must poll the MB_MASTER or MB_SLAVE
instructions for transmit and receive complete conditions.
If a port is to respond as a slave to a Modbus master, then that port cannot be used by
MB_MASTER. Only one instance of MB_SLAVE execution can be used with a given port.
Likewise, If a port is to be used to initiate Modbus master requests, that port cannot be used
by MB_SLAVE . One or more instances of MB_MASTER execution can be used with that
port.
If your program operates a Modbus slave, then MB_SLAVE must poll (execute periodically)
at a rate that allows it to make a timely response to incoming requests from a Modbus
master.
If your program operates a Modbus master and uses MB_MASTER to send a request to a
slave, then you must continue to poll (execute MB_MASTER) until the response from the
slave is returned.
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