Protocol descriptionΒΆ
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).
The MODBUS operating mode of a serial interface is set in the PLC configuration: Setting COMx - MODBUS
MODBUS client
In this operating mode, the telegram traffic with the server(s) is handled via the Function Block COM_MOD_MAST: COM_MOD_MAST
This Function Block sends MODBUS request telegrams to the server(s) via the set interface and receives MODBUS response telegrams from the server(s) via this interface.
The MODBUS blocks transferred by the server contain the following information:
- MODBUS address of the interrogated server (1 byte)
- Function code that defines the request of the client (1 byte)
- Data to be exchanged (n bytes)
- CRC16 control code (2 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 |
| 07 | 07 | Fast reading the status byte of the CPU |
| 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 |
| 07 | 07 | 8 bits |
| 15 | 0F | 1968 bits |
| 16 | 10 | 123 words / 61 double words |
| 22 | 16 | Write: 1 word |
| 23 | 17 | Read: 125 words / 62 double words Write: 120 words / 60 double words |