t information
frequency
his chapter describes the communication protocol.
he provide
Communication Protocol
128
9
Communication Protocol
9.1 What this chapter contains
T
T
RS485
communication interface.
It adopts international
standard MODBUS communication protocol to perform master-slave communication. The user can
realize centralized control through PC/PLC, upper control PC, etc. (set the control command,
running
, modify relevant function codes, monitor and control the operating
state and faul
and so on) to adapt specific application requirements.
9.2 Brief instruction to MODBUS protocol
MODBUS protocol is a software protocol and common language which is applied in the electrical
controller. With this protocol, the controller can communicate with other devices via network (the
channel of signal transmission or the physical layer, such as RS485). And with this industrial
standard, the controlling devices of different manufacturers can be connected to an industrial
network for the convenient of being monitored.
There are two transmission modes for MODBUS protocol: ASCII mode and RTU (Remote Terminal
Units) mode. On one MODBUS network, all devices should select same transmission mode and
their basic parameters, such as baud rate, digital bit, check bit, and stopping bit should have no
difference.
MODBUS network is a controlling network with single-master and multiple slaves, which means
that there is only one device performs as the master and the others are the slaves on one
MODBUS network. The master means the device which has active talking right to send message
to MODBUS network for the controlling and inquiring to other devices. The slave means the
passive device which sends data message to the MODBUS network only after receiving the
controlling or inquiring message (command) form the master (response). After the master sends
message, there is a period of time left for the controlled or inquired slaves to response, which
ensure there is only one slave sends message to the master at a time for the avoidance of singles
impact.
Generally, the user can set PC, PLC, IPC and HMI as the masters to realize central control. Setting
certain device as the master is a promise other than setting by a bottom or a switch or the device
has a special message format. For example, when the upper monitor is running, if the operator
clicks sending command bottom, the upper monitor can send command message actively even it
cannot receive the message from other devices. In this case, the upper monitor is the master. And
if the designer makes slave.
The master can communicate with any single slave or with all slaves. For the single-visiting
command, the slave should feedback a response message; for the broadcasting message from the
master, the slave does not need to feedback the response message.
