Slaves do not initiate messages on their own, they only respond to queries from the master.Ī master’s query will consist of a slave address (or broadcast address), a function code defining the requested action, any required data, and an error checking field. Slaves return a response to all queries addressed to them individually, but do not respond to broadcast queries. Masters can address individual slaves, or can initiate a broadcast message to all slaves. Other devices may function as both clients (masters) and servers (slaves). The Acromag I/O Modules form slave/server devices, while a typical master device is a host computer running appropriate application software. A slave is any peripheral device (I/O transducer, valve, network drive, or other measuring device) which processes information and sends its output to the master using Modbus. The other devices (slaves/servers) respond by supplying the requested data to the master, or by taking the action requested in the query. Modbus devices communicate using a master-slave (client-server) technique in which only one device (the master/client) can initiate transactions (called queries). Modbus is now a widely-accepted, open, public-domain protocol that requires a license, but does not require royalty payment to its owner. It has since become an industry standard method for the transfer of discrete/ analog I/O information and register data between industrial control and monitoring devices.
The Modbus protocol was developed in 1979 by Modicon, Incorporated, for industrial automation systems and Modicon programmable controllers.
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