MODBUS RTU Connection Details

The MODBUS RTU protocol is implemented in the AC500 Processor Modules.

MODBUS is a master-slave (client-server) protocol. The client sends a request to the server(s) and receives the response(s).

Available serial interfaces can work as MODBUS interfaces simultaneously.

The MODBUS client operating mode of an interface is set with the Function BlockCOM_MOD_MAST: COM_MOD_MAST

Technical data

The MODBUS operating mode and the interface parameters are set in the PLC configuration: Setting COMx - MODBUS

Description of the MODBUS protocol:

Supported standard	PM55x and PM56x: EIA RS-485 PM57x, PM58x and PM59x: EIA RS-232 / RS-485
Number of connection points	1 client Max. 1 server with RS-232 interface Max. 31 servers with RS-485
Protocol	MODBUS
Operating mode	Client/server
Address	Server only
Data transmission control	CRC16
Data transmission speed	Up to 187.500 baud
Encoding	1 start bit 8 data bits 1 parity bit, (optional) even, odd, mark or space 1 or 2 stop bits
Max. cable length for RS-485	1.200 m at 19.200 baud

If a Processor Module provides more than one serial interface, both interfaces (COM1/COM2) can be operated simultaneously as MODBUS interfaces and can operate as MODBUS server as well as MODBUS client.

Bus topology

Point-to-point with RS-232 or bus topology with RS-485. MODBUS is a master-slave protocol.

For further information on MODBUS see chapter Communication with: Communication \ with MODBUS RTU