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PLC Gremlin
March 25th, 2005, 12:48 PM
Hey All,

Im having a difficult time understanding the communication command
on the setup screen of the MSG Instruction.
I have 6 MicroLogix 1500's and a PV on a DH-485 network.

What I do not understand is the 500CPUWrite vs. the 500CPURead .
From the posts I have read and other literature, it seems that you have to program a Write Instrction in one PLC(Node1) and if your sending data to (Node2) then you have to program a Read Instruction in that PLC....
I done some testing yesterday and triggered my MSG in PLC1 to send data to Node 2 (PLC2). I do not have a MSG Read instruction set up in PLC2. When I went online with PLC 2, the word data that I sent was there. SO my question is , What is the Read command for?

Ron Beaufort
March 25th, 2005, 01:32 PM
it seems that you have to program a Write Instrction in one PLC(Node1) and if your sending data to (Node2) then you have to program a Read Instruction in that PLC....



thatís incorrect ... actually you only need to program the PLC at ONE end of the line ... not both of them ...



basically ... if you want the PLC that you are programming to SEND the message, then use the WRITE configuration ... you donít need to do anything to the other PLC ...



but ... if you want the PLC that you are programming to RECEIVE the message, then use the READ configuration ... again, you donít need to do anything to the other PLC ...

What is the Read command for?

basically you can use it to retrieve (go get) information from another PLC on the network and bring that information into the PLC that you are currently programming ...

Bruce99
March 25th, 2005, 02:23 PM
Ron is correct, and when programming in X PLC and you need a number that is Y PLC do you go to Y and program a write or just "read" the number from X PLC. It is a matter of convience. In some cases tho there may be an extra word in PLC Y that is already being sent to X. When I program write MSGs and they are small blocks, under 10 words, Ill put 2 or 3 extra in for future considertion. You must document this or it will be long forgoten. A small pencil is better than a long memory :)
Regards Bruce .....

PLC Gremlin
March 25th, 2005, 10:47 PM
Great, That was what I was hoping for... and it makes since, seeing that my Write worked when I tested It. OK, now since you guys have told me how the read works, I have backed up and looked at my problem from a different angle. I have to get 200 integer words from PLC1 and copy it to each of the other 5 PLCs. Right now I have it set up like so. A "Save" button in my panelvie.. hang on, let me just see if I can give you a pic of my rung...
http://www.plctalk.net/qanda/uploads/MSG-Rung.JPG

Right now when b3:10/1 is triggered from pv, I latch this rung and then use MG12:0 and MG12:1 to write 103 words each to PLC2.
Then MG2 and MG3 write to PLC3 and so on.... Anyway, when I read what you guys were saying about the READ command I thought it might be easier to let the PV trigger a rung in each PLC and let all 5 PLC's go get the 206 words from PLC1 individually. Would that be more reliable than having PLC1 try to write all these messages sequentially.... The next question is; what is the easiest way to Get the PV button to trigger a bit in each PLC. As far as I know, One PV button can only address one TAG which can only be addressed to one NODE on ONE PLC. DO I stack up buttons on top of each other in PV... Any other suggestions on doing this would be appreciated... Thanks everyone.

Fred Floggle
March 26th, 2005, 05:04 AM
As mentioned , the AB MSG function is passive - you don't have to do anything at the "other end" to let it happen , though you might want to group data logically to make the transfer easier .
You will note that the above messages are cascaded to prevent them all bumping into each other - I also like to use a poling timer - you will find that for some transfers , it is not necessary to do a MSG if nothing changes , and hence not necessary to bung the network up .
I'm not sure I like the idea of keep repeating the MSG in the event of error , I tend to retry say 5 times then generate an alarm - there is no point leaving the MSG filling the network with traffic and possibly critical data not getting transmitted

Doug_P
March 26th, 2005, 06:10 AM
The next question is; what is the easiest way to Get the PV button to trigger a bit in each PLC.

I would have the PV send a single bit or word value which triggers PLC1 to send (WRITE) a single word message to each of the others which need data. They in their turn will then initiate their own required READ messages.

.02