Lab UPDATE- DH+ Communications



To explore to and between PLCs.


Pre-Lab: (due at the start of lab period)

1. Write two programs to run on a PLC-5 and a Softlogix PLC. When a button is pushed on one PLC (node #1), it should send data to another PLC to turn on an output. The message should be passed using the DH+ network. Write a second program that uses DH+ to run on a PLC so that when a button is pushed it requests data to set an output.

2. Write a program to send a message out the RS-232 port on the SoftLogix (to a connected PC running a terminal program) when an input is active.


Process Description:

As with the previous use of analog input and output cards, we need to set up blocks of memory that contain communication information. These blocks indicate what is to be sent and where.

The Data Highway Plus, DH+, network uses a single path to connect numerous devices. To use this the block of memory below must be used to set up the information to be sent or received.




When PLCs communicate, one PLC must write contents of its memory to a second, or one PLC must request contents of memory from a second. The program below shows the basic steps involved in communication.






RS-232 Communication cables

Data Highway cables

Screw drivers

2 Computers



2. (single teams) Have two computers available. One will be used for programming the PLC, and the other will be used as an ASCII terminal. Enter and download the program to the PLC as normal. Disconnect the serial cable, and connect it to the other PC. Run ‘hyperterm’ and test the program.



1. Pre-lab and updated laboratory materials.




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