GE GFK-1541B Troubleshooting a Ladder Program, Commreq Status Word is Zero 0 and FT Output is OFF

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Troubleshooting a Ladder Program

As mentioned, there are several forms of status data which can be used in your ladder program. The use of the LAN Interface OK bit in the LAN Interface Status Word was described in the ladder program fragment above. Some of the status data can be used to troubleshoot your program in its developmental stage. The two primary sources of this data are the FT Output on the COMMREQ Function Block and the COMMREQ Status word (CRS word).

FT Output is ON

If after executing a COMMREQ Function, the FT Output is ON, then there is a programming error in one or more of the following areas.

ƒInvalid rack/slot specified. The module at this rack/slot is unable to receive a COMMREQ Command Block.

ƒInvalid Task ID. For the Series 90-30 Ethernet Interface this value should always be zero.

ƒInvalid Data Block length (0 or greater than 128).

COMMREQ Status Word is Zero (0) and FT Output is OFF

If after executing a COMMREQ function, the CRS word is zero (0) and the FT Output is OFF, then the Command Block has been sent to the Ethernet Interface, but no status has been returned yet. If this condition persists, check the PLC Fault Table for information.

COMMREQ Status Word is Not One (1)

If after executing a COMMREQ function, the CRS word is not one (1) indicating success, then there were:

ƒErrors in the Command Block (the Channel command code or parameters), or

ƒThe command parameters were valid but there was an error in completing the request.

If the CRS word does not contain a 1 indicating success, then it contains either a 0 or a code indicating what error occurred.

GFK-1541B Chapter 4 Programming Modbus/TCP Channel Commands 4-33