Experimental Physics and
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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,
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ANJ, 10 Nov 2011 |
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