RS Logix 5000 Generic Ethernet Module Configuration Limits?

[Chris Elston 79]

I have a question. I am setting up an Anybus Ethernet/IP Gateway. The maximum transmission of this device is 512 Bytes Input and Output. I don't need the inputs, 32 bytes is fine, however I would like to max out the outputs, to get what I paid for sort of speak..I want the full 512 Bytes on the output size. However, in the RS Logix 5000 software, it won't let me have anything larger that 496 Bytes configured. SINT 8-bit format is fine for what I need and it doesn't seemed to effect it if I choose INT or DINT, the byte size is the same based on byte, word or double word format. Does anyone know where this limitation comes from? 497 bytes yield an error.

[Sergei Troizky 26]

496=512-16 Status word?

[Chris Elston 79]

I dunno. I don't have a status word configured. And it's not connected to the Anybus Gateway, so it's not like it "senses" the gateway and would know it's technical limits. It seems more like it's something to do with RS Logix or some "rule" I haven't heard about or learned yet. But I like your theory!

[Ken Roach 194]

I think what you are seeing is the maximum CIP connection size and the inclusion of the Sequence Word (two bytes). I know that EtherNet/IP inherited the CIP connection size rules from ControlNet (and really, those are inherited from the ControlLogix backplane design) so the maximum data payload is 512 bytes. Every output connection needs to include a Sequence Word that changes each time the data is Produced, so those 16 bits are subtracted from the maximum data payload, to yield a usable-by-the-tag-database size of 496 bytes. There's also a Run/Idle header (4 bytes) that is sent along with the Output data, but it's part of the connection overhead and therefore isn't subtracted from the usable I/O data size. This info comes from hazy memories of the CIP standard and from an old RA developer's whitepaper (io_clogix1.0.pdf) that I can't seem to find on the Internet anymore. E-mail me if you'd like a copy.

[Chris Elston 79]

Thanks Ken! Perfect, and makes sense.

[billysmithde2 1]

I don't think the 496 bytes is per spec - it's less. Other products can do more in a Class1 connection - see picture attached. I think the limit is 511 bytes, and I count like this: T->O 511bytes = 2byte sequence count + 509bytes data O->T 511bytes = 2byte sequence count + 4byte run idle header + 505bytes data The 511byte limit might come from section 3-5.5.1.1 of CIP Volume 1, Edition 3.12: Connection Size (in bytes) ... is 9bits in the Forward Open: 2^9 = 512, so range would be 0-511 <- max data size Connection Size: The maximum size, in bytes, of the data for each direction (where applicable) of the connection. For a variable sized connection, the size shall be the maximum size of the buffer for any transfer. The actual size of the transfer for a variable connection shall be equal to or less than the size specified for the network connection. The maximum buffer size shall be dependent on the links that the connection traverses. The connection size includes the sequence count and the 32-bit real time header, if present.