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4-34 TCP/IP Ethernet Communications User’s Manual – January 1996
GFK-1004B
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 Program
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 or contact. 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 (CRS) word, and OK
and FT Outputs 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.