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4-26 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual
–
May 2002 GFK-1541B
4
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. Topics discussed are:
Essential Elements of the Ladder Program
Troubleshooting Your Ladder Program
Monitoring the Communications Channel
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 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 the 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.
The example ladder program segment starting on the next page illustrates how to incorporate
these important points in your program.