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<== Date ==> <== Thread ==>

Subject: Re: Ethernet/IP Device Support and CompactLogix
From: Kay-Uwe Kasemir <[email protected]>
To: Mike Forster <[email protected]>
Cc: John Dobbins <[email protected]>, EPICS Tech-Talk <[email protected]>
Date: Fri, 29 Jun 2007 17:01:04 -0400
Hello:

Thanks for tracking this down!

500 is a value that happens to work right now for all the ControlLogix PLCs
at the SNS, but that is based on luck, not a full understanding of the real limit.


The header file ether_ip.h has some comments
about my (failed) attempts to figure this buffer limit business out.
I found no specific buffer limit in the protocol specification.
Tests with an AllenBradley ControlLogix PLC went like this:
When reading elements of an SINT array, I could pick the
_response_ packet size pretty much to the byte by asking
for more or fewer array elements (SINT==8bit byte).
From that, it seemed that the PLC could send a response
of 538 bytes, not one byte more.

So in that case, it seemed obvious that the PLC _response_ was
limited to 538 bytes, while you now found a _request_ limit of 548.

It's been unclear to me if the limit of 538 in the response
applies to the total byte count for the whole
protocol wrapper + status codes + meta info + actual data,
or if some of the protocol wrappings don't count.

-Kay

On Jun 29, 2007, at 16:21 , Mike Forster wrote:

Hi folks,
Just an update on our EtherIP with CompactLogix use:
We are communicating smoothly with the PLC at this point after sorting out an anomaly.


We saw that when a record scan list became long (too many records with the same scan rate), all the elements in that scan list would stop responding. Eventually we traced this down to the EIP_buffer_limit which was the default 500. This limit at 500 allows data to be sent that is 554 bytes long. After some trial and error we found that the PLC responded correctly if the data sent was 548 bytes long or less. When we set EIP_buffer_limit to 494 our problems went away.

Mike Forster with John Dobbins
LEPP
Cornell University


References:
Re: Ethernet/IP Device Support and CompactLogix Mike Forster

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