PLC communication RS500

bop · Jul 5, 2005

Hi there we have several allen bradley SLC connected to each other i am just wondering if i add another analog device on one of the plc how can i make the other plc to see the data fom that plc.and also with that of the discrete inputs. I want to know more about Data File addressing can some give info on that.thanks.

Ken Moore · Jul 5, 2005

A good place to start is with the instruction set reference, after you've read that, come back with more specfic questions.
Here's the link:

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1747-rm001_-en-p.pdf

JesperMP · Jul 5, 2005

What kind of SLC ? (SLC5/01, SLC5/03, SLC5/04, SLC5/05) ?
What kind of network do you have presently ? (none, DH485, DH+, Ethernet, other..).

bop · Jul 7, 2005

SLC 5/03's and SLC 5/04's using DH485

Ken Moore · Jul 7, 2005

Generally speaking, to minimize network traffic, in the PLC with the analog value, map the value to a dedicated message file, such as N:37 or such, you can then map the discrete I/O to a register in the message file also.

Assuming your analog value is stored in N7:0, use the MOV instruction to place the value in N37:0, then assuming your discrete inputs are I:2.0 use the MOV again to place the values into N37:1, then use the MSG instruction to transfer the values in N37 from one plc to the other. Read the help files on the MOV and MSG instructions. I would really recommend you download the instruction reference manual, it goes into a lot more depth on each instruction than the online help file does.

JesperMP · Jul 7, 2005

One comment:
At 19200 baud, DH485 isnt the most potent networking system out there.
You should try to limit the networked data to only what is really needed, and only as often as is really required.
Dont try to transmit everything to every node. Separate the "Nice to haves" from the "Must haves".

chief weegum · Jul 7, 2005

If it's the only thing he would transfer, limit the networked data is not absolutly essential.

JesperMP · Jul 21, 2005

chief weegum said:
If it's the only thing he would transfer, limit the networked data is not absolutly essential.

bop said:
how can i make the other plc to see the data fom that plc.and also with that of the discrete inputs

He wants to share both data from the PLC memory files, as well as the i/o.

"Only" data from the PLC internal memory AND discrete i/o.

TConnolly · Jul 21, 2005

I might be up in the night here, but...

I have always thought that it is better to use a READ MSG instruction in the PLC that wants the data instead of a WRITE MSG in the PLC that provides the data. My thinking is that this will eliminate comm traffic because the receiving PLC only gets the data when it wants it, whereas if I use a WRITE MSG then I might be sending data that is not needed by the receiver.

Is this correct? Or does it matter?

msinclair · Jul 21, 2005

There was a thread a while back (I can't find it at the moment) where someone actually studied the time differential and found it to be not very significant.

I personally prefer the READ's because I think it is better for the sake of documentation. No "mystery" data appearing in a "written to" PLC . . .

Marc

TConnolly · Jul 21, 2005

msinclair said:
I personally prefer the READ's because I think it is better for the sake of documentation. No "mystery" data appearing in a "written to" PLC . . .

Marc

That the best reason I have heard yet for doing it that way.

Ron Beaufort · Jul 21, 2005

sort of kind of off topic ...

disclaimer: I know that this won’t work for our poster’s problem - since he mentioned using SLC-5/03s and SLC-5/04s on DH-485 ...

but it’s kind of interesting so I thought I’d add it to this thread just for kicks ...

if you only need to transfer one 16-bit word of information over DH+, then the “Global Data” approach will let you do it for “free” ... specifically, the processor will automatically transmit one word of information each time it polls the DH+ network ...

this should give enough of a starting point for anyone who wants to do any research on the technique ... unfortunately it is only applicable to systems communicating on DH+ (examples: SLC-5/04 and PLC-5 processors) ...

msinclair · Jul 21, 2005

msinclair said:
There was a thread a while back (I can't find it at the moment) where someone actually studied the time differential and found it to be not very significant.

Well - since I couldn't find the thread that I remembered, I looked up some old notes from one of my AB training seminars and I found this quote in reference to Reads and Writes on DH+:

In most cases, a message write will always work faster than a message read because the data is compiled into the original message.

In the case of the token passing network (like DH+), this is even more true because the response can't be sent until the 2nd node has the token. So - the "effect" of the write command will be complete almost "immediately", but the effect of the read command has to wait for the 2nd PLC to get the token after it has had time to compile the response!

Marc

PLC communication RS500

bop

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Ken Moore

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JesperMP

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bop

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Ken Moore

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JesperMP

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chief weegum

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TConnolly

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msinclair

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