PLC-2: unsolicited messages

An unsolicited message is generated in a PLC-2 using a message zone. This involves programming several rungs of ladder since the PLC-2 does not have a message instruction. These rungs were programmed following the example in the Allen-Bradley 1771-KA2 User's Manual, Chapter #5. In this example a PLC-2 connected to a 1771-KA2 at station 21 on a DH sends a message to a KF2 at station 11. This is the Header rung.

The first GET instruction contains the address of the KA2 that the PLC-2 is connected to, 021. The second GET contains the address of the error code storage word. The third GET is the timeout preset code. This is the Command rung.

The start bit initiates the message. The command code is indicated by the address of the second XIO instruction, 011/03, where 011 is the remote station address, and 03 indicates a normal unprotected write message. The first GET contains the starting data table address for the remote station, this is where the data will be written. This is the Delimiter rung.

The preceding rungs represent the message zone. The following rungs timer initiate the message. This rung checks for faults in the communication.