User Manual

Easy to communicate between devices
7.10 PtP, USS, and Modbus communication protocols
Easy Book
168 Manual, 03/2014, A5E02486774-AF
Table 7- 15 Modbus instructions
Type of communication
Instruction
Modbus RTU (RS232 or RS485) MB_COMM_LOAD: One execution of MB_COMM_LOAD is used to set up PtP port
parameters like baud rate, parity, and flow control. After the CPU port is configured
for the Modbus RTU protocol, it can only be used by either the MB_MASTER or
MB_SLAVE instructions.
MB_MASTER: The Modbus master instruction enables the CPU to act as a Modbus
RTU master device and communicate with one or more Modbus slave devices.
MB_SLAVE: The M
odbus slave instruction enables the CPU to act as a Modbus RTU
slave device and communicate with a Modbus master device.
Modbus TCP (PROFINET) MB_CLIENT: Make client-server TCP connection, send command message, receive
response, and control the disconnection from the server.
MB_SERVER: Connect to a Modbus TCP client upon request, receive Modbus
message, and send response.
The Modbus instructions do not use communication interrupt events to control the
communication process. Your program must poll the MB_MASTER / MB_SLAVE or
MB_Client / MB_Server instructions for transmit and receive complete conditions.
A Modbus TCP client (master) must control the client-server connection with the
DISCONNECT parameter. The basic Modbus client actions are shown below.
1. Initiate a connection to a particular server (slave) IP address and IP port number
2. Initiate client transmission of Modbus messages and receive the server responses
3. When required, initiate the disconnection of client and server to enable connection with a
different server.