Modbus®RTU

The MODICON Modbus® RTU protocol is implemented in the AC500 Processor Modules.

Numerous automation devices, such as PLC installations, displays, variable-frequency inverters or monitoring systems, for instance, have a Modbus® RTU interface by default or as an option and can therefore communicate with the AC500 Processor Modules without any problems via the serial interfaces COM1 and COM2 (RS-485 at PM55x and PM56x; RS-232 or RS-485 at PM57x, PM58x and PM59x).

Modbus® is a master-slave protocol. The server sends a request to the client and receives its response.

Both interfaces COM1 and COM2 can work as Modbus® interfaces simultaneously.

The Modbus® operating mode of an interface is set with several function blocks.

Technical data

Both serial interfaces of the Processor Modules can be operated simultaneously as Modbus interfaces and can operate as Modbus server as well as Modbus client.

The Modbus operating mode and the interface parameters are set in the PLC Configuration (Controller configuration / Modbus AC500 PLC Configuration).

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 server Max. 1 client with RS-232 interface Max. 31 clients with RS-485
Protocol	Modbus® (server/client)
Data transmission control	CRC16
Data transmission speed	Up to 187.500 baud
Encoding	1 start bit 8 data bits 1 parity bit, even or odd (optional) 1 or 2 stop bits
Max. cable length for RS-485	1.200 m at 19.200 baud

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 Modbus RTU” Communication \ with Modbus® RTU and Modbus® TCP/IP.