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GFK-1541B Chapter 3 Programming Channel Commands 3-43
3
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
Managing Channels and TCP Connections
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 transitional contact for
Series 90-70 PLCs). 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 OK
(Series 90-70 PLCs only) 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.
The example ladder program segment starting on the next page illustrates how to incorporate
these important points in your program.
COMMREQ Example
In the example logic that follows, the input values for the Block Move
Functions are taken from the Establish Read Channel (2003) command
Example 1 in this chapter.
Nicknames are used in this example to make the ladder program easier to
follow. LANIFOK is bit 16 of the LAN Interface Status bits. All other
nicknames can be assigned as you desire.