Protocol description

The MODBUS TCP 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).

Each ETHERNET interface can work as MODBUS client and server interface in parallel if required.

The MODBUS operating mode of an ETHERNET interface is set in : MODBUS Protocol

MODBUS client

In this operating mode, the telegram traffic with the server(s) is handled via the Function Block ETHx_MOD_MAST: ETHx_MOD_MASTMODBUS request telegrams to the server(s) via the set interface and receives MODBUS response telegrams from the server(s) via this interface.

The MODBUS Function Blocks transferred by the client contain the following information:

• Transaction identifier for synchronization between messages of server and client (2 byte)
• Protocol identifier (0 for MODBUS/TCP) (2 byte)
• Length field (Number of bytes in frame) (2 byte)
• Unit identifier (1 byte)
• Function code that defines the request of the client (1 byte)
• Data to be exchanged (n bytes)

MODBUS server

In this operating mode, no Function Block is required for MODBUS communication. Sending and receiving MODBUS telegrams is performed automatically.

The AC500 CPUs process the following MODBUS operation codes:

Function code		Description
DEC	HEX
01 or 02	01 or 02	Read n bits
03 or 04	03 or 04	Read n words
05	05	Write one bit (encoded in one word)
06	06	Write one word
15	0F	Write n bits (encoded in one byte)
16	10	Write n words
22	16	Mask write
23	17	Read/write multiple words in one telegram

The following restrictions apply to the length of the data to be sent:

Function code		Max. length
DEC	HEX
01 or 02	01 or 02	2000 bits
03 or 04	03 or 04	125 words / 62 double words
05	05	1 bit
06	06	1 word
15	0F	2000 bits
16	10	123 words / 61 double words
22	16	Write: 1 word
23	17	Read: 125 words / 62 double words Write: 123 words / 61 double words