Chapter 9. Modbus/TCP Client
GFK-2224Q January 2017 165
7
8
9
Figure 60: COMMREQ Ladder Logic Segment (continued)
Rung # 7: When OPEN_SUCCESS is set it triggers READ_REQ, which enables execution of the BLKMOV, MOVE
and COMMREQ functions for the Modbus/TCP Read COMMREQ. READ_REQ is a one-shot (Positive Transition)
coil, activating once when OPEN_SUCCESS transitions from OFF to ON.
Rung # 8: The MOVE WORD function moves a zero to the CRS word referenced in the Command Block (see rung
#9). This clears the CRS word. This rung also resets the READ_FLT output coil of the COMMREQ Function Block
in rung #10.
Rung # 9: The BLKMV INT functions set up the COMMREQ Command Block contents. When this rung is
activated, the constant operands are moved into the memory beginning at the address indicated in the
instruction. The constant operands in this example are defined in the Modbus/TCP Read Example in this
chapter.
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