GE 90-30 PLC Section 5: Controlling Communications in the Ladder Program, Essential Elements of the Ladder Program

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GFK-1084B Chapter 3 Programming Communications Requests
Section 5: Controlling Communications in the Ladder Program
This section provides tips on how to control communications in your ladder program.
Only segments of actual ladder logic are included. For a sample of a complete working
program, see Appendix D. Topics discussed are:
Essential Elements of the Ladder Program
Troubleshooting Your Ladder P rogram
Monitoring Communications in the Ladder Program
Sequencing Communications Requests
Essential Elements of the Ladder Program
Every ladder program, whether in the developmental phase or the operational phase,
should do the following before initiating a COMMREQ function.
1. Initiate the COMMREQ function with a one-shot transitional coil. This prevents
accidentally sending the same COMMREQ Command Block more than once.
2. Include at least the LAN Interface OK bit in the LAN Interface Status Word as an
interlock contact for the COMMREQ function. You may choose to add more
interlocks.
3. Zero the word location you specify for the COMMREQ Status word (CRS word) and
the FT Output of the COMMREQ Function Block before the COMMREQ function is
initiated.
4. Move the command code and parameters for the Channel Command into the
memory location specified in IN input of the COMMREQ Function Block before the
COMMREQ function is initiated.