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Chapter 9. Modbus/TCP Client
GFK-2224Q January 2017 141
Status Data
There are several types of status available to the client application program.
LAN Interface Status Bits (LIS Bits): The LIS bits comprise bits 1–16 of the 80-bit status area. The location of
this 80-bit status area is assigned using the configuration software. The LIS bits contain information on the
status of the Local Area Network (LAN) and the Ethernet interface.
Channel Status Bits: The Channel Status bits comprise bits 17–80 (64 bits) of the 80-bit status area. When
used for Modbus/TCP channels, these bits consist of a
connection open
bit and an unused bit, reserved for
future use, for each of the 16 channels that can be established. Status bits for unused channels are always set
to zero.
COMMREQ Status Word (CRS Word): The 16-bit CRS word will receive the initial status of the communication
request. The location of the CRS word is assigned for each COMMREQ function in the COMMREQ Command
Block.
FT Output of the COMMREQ Function Block: This output indicates that the PLC CPU detected errors in the
COMMREQ Function Block and/or Command Block and did not pass the Command Block to the Ethernet
interface.
The Logic Program Controlling Execution of the COMMREQ Function Block
The COMMREQ must be initiated by a one-shot to prevent the COMMREQ from being executed repeatedly each
CPU scan, which would overrun the capability of the Ethernet interface and possibly require a manual restart.
Checking certain status bits before initiating a COMMREQ function is also important. In particular, the LAN
Interface OK bit should be used as an interlock to prevent execution of the COMMREQ function when the
Ethernet interface is not operational. Following initiation of a COMMREQ on a channel, no further COMMREQs
should be issued to that channel until a non-zero CRS word has been returned to the program from the
Ethernet interface.