comunicate between 2 allen bradley plcs

[cooter1955 0]

i have 2 allen bradley slc 5/03 on a local DH-485 network and i need to make a input on plc #1 turn on an output on plc #2, which instruction would do this? i'm not a whiz at programming but i understand the basics. Thanks very much...cooter1955

[Mickey 71]

The best solution is to hard wire it. If you can't do that and must do this across the network then use a message instruction in PLC#2 to read the status of the input from PLC#1. Then use some logic in PLC#2 to control the output. Don't try to control the output directly. Something like this. B3:0/0-------- O:0.0/0 --| |--------------( )---- Edited 12 Mar 2010 by Mickey

[Bill Linne 0]

It depends on how quickly you need the output (of PLC #2) to respond to PLC #1. A Read MSG in PLC #2 will work, but it means PLC #2 is constantly asking PLC #1 when to energize the output. The other way to do it is to have PLC #1 Write a MSG to PLC #2 at the instant PLC #2 needs to change the output condition. This is known as "report by exception". You will MSG at least one word. So, create a timer in PLC #1 set for 100ms (or whatever is appropriate for the reaction time you desire), and every time the timer is DN, it MOV's the word you will MSG to an unused word in the SAME PLC. Next rung, if the two words are not equal (NEQ), trigger the Write MSG to PLC #2. (Every 100ms you are checking to see if the word to be messaged has changed. If it has (and is therefore not equal to the copy made 100ms ago), send the change to PLC #2.) Then, as a parallel branch at the input of the MSG rung, use an bit from the internal clock to also send the MSG (say once per minute (or whatever is appropriate)). This allows a periodic update whether the word changed or not, but is infrequent enough to no hog network bandwidth. Then in PLC#2 monitor the bit(s) of the word of interest to control the output. Bill

[Mickey 71]

Good point, I didn't think of that. I generally read not write. But in this case it is probably better to write. I still think hard-wired would be even better, if at all possible.

[BobLfoot 302]

For a simple application such as this {only 2 plc, only 1 bit to message} Bill you are spot on that a write is the most efficient. When the situation becomes more complex, ie. more plcs or more signals to handshake then a Message Read is 2x faster than a Message Write just because of fewer traffic cycles. I've seen a network of 5 slcs and 2 PV's go from having a single bit looptime of 12 seconds down to under two by elimianting all message writes and using only reads.

[cooter1955 0]

Thanks Guys, I've enjoyed listening and now I have a head full of ideas. I'll try a MSG read in plc #2 to monitor plc #1's input. I have three outputs that need input from plc #1 and they are just push a button and start a pump situations, timing should not be an issue. Thanks again....cooter1955