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:

ƒEssential Elements of the Ladder Program

ƒTroubleshooting Your Ladder Program

ƒMonitoring the Communications Channel

ƒManaging 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.

GFK-1541B

Chapter 3 Programming Channel Commands

3-43

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GE GFK-1541B Controlling Communications in the Ladder Program, Essential Elements of the Ladder Program, Commreq Example