Yaskawa VS-616G5 Modbus Plus Communication Card - Manual (Page 23)

5-2 Modbus Plus MSTR Function
Modbus Plus MSTR Function
When using a Modicon programmable logic controller to access registers from a node on the MB+
network, an MSTR function block must be used. All Modicon controllers supporting the Modbus
Plus communication protocol have a MSTR (Master) function block. The MSTR function block is
used to initiate Modbus Plus message transactions. Each type of network transaction has an
associated operation code. The Modbus Plus transactions supported by the VS-616G5 drives
are:
MSTR Function Operation Code
Write Data 1
Read Data 2
Write Global Data 5
Read Global Data 6
The MSTR function block is a three section function block as shown below:
The MSTR function block has two control inputs. The ENABLE input enables the instruction when
it is ON. The TERMINATE input terminates the active operation when it is ON.
The MSTR function block can produce three possible outputs. The OPERATION ACTIVE output
goes ON while the instruction is active. The OPERATION TERMINATED output goes ON if an
error occurs during the transaction or if the MSTR operation is terminated prior to completion.
The OPERATION SUCCESSFUL output goes ON when an MSTR operation has been completed
successfully.
When inserting a MSTR function block into the ladder logic, three pieces of data are required:
1. Control Block starting address
2. Data Block starting address
3. Maximum number of Data Block registers
Control Block
Starting Register
Data Block
Starting Register
MSTR
Area Size
ENABLE
TERMINATE
OPERATION ACTIVE
OPERATION TERMINATED
DUE TO ERROR
OPERATION COMPLETED
SUCCESSFULLY