MODBUS, Instrukcje, Allan Bradley
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MODBUS
Table of Contents
MODBUS enables you to establish master-slave communications with any connected
device that supports the MODBUS protocol. Any controller in the network may function as
either master or slave using any of the controller's existing Com ports.
Unitronics currently supports RTU (binary) transmission mode.
Using MODBUS
Before using a MODBUS operation in your application, you must:
Synchronize the communication port settings of master and slave devices. This is
done by placing Com Port Init FBs, set with identical parameters, in the ladder
application of both master and slave.
Include at least 1 MODBUS Configure FB in the ladder application of both master and
slave. The port you select must be the same port selected in the Com Port Init FB.
Enable slave devices to be accessed by placing a Scan FB in the slave's Ladder
application.
The figure below shows the elements required to carry out a Read Coils Operation.
1
Note that the operand addresses in slave PLCs are indirect addresses (pointers).
MODBUS Operations
The MODBUS FBs are grouped under MODBUS on the FB's menu.
2
3
MODBUS: Configuration
A MODBUS Configuration FB must be included in both master and slave Ladder
applications as shown below.
Parameter
Type
Function
Port Number
Constant
Click the drop-down arrows to view available ports; click the port you
want to use.
Network ID
Constant
This number identifies the device on the network. You can assign any
number from 0-255. Do not assign the same ID number to more than
one device.
Time out
Constant
or MI
This is the amount of time a master device will wait for an answer from
a slave. Time out units are defined in 10 msecs; a Time out value of
100 is equal to 1 second.
Retries
Constant
or MI
This is the number of times a device will try to send a message.
Function in
Progress
MB
This bit is ON when MODBUS is active. Use this as a condition bit for
MODBUS operations to avoid communication conflicts.
4
The Ladder application below enables the controller act as a MODBUS master and read
coils in a slave PLC. The Scan operation in the final net enables the controller to also act
as a slave.
5
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