CIP question

[Ballen22 0]

Hello Everyone, I was wondering if anyone knew the maximum block size (or the maximum number of words of data) one CIP WRITE/READ can send through EIP using one packet? For instance how much information can I send without having to send multiple packets. I appreciate the help. Thank you

[Ken Roach 194]

My recollection from CIP training classes is that EtherNet/IP retained the ControlNet maximum data payload of 510 bytes. I know I've set up MSG instructions that moved more than that, and the controller automatically broke it up into multiple write packets.

[Mark- 17]

Hello, My notes have (bytes) : 511 - the overhead of 28 per frame/packet. Of course the complete frame/packet is larger because of the wrapping layers. Are you creating a 'driver' or using PLC instructions for the operations? As Ken says, if the latter I am not sure the packet size is a limiting factor. If the former, the packet size is of importance. Edited 2 Jul 2014 by Mark-

[billysmithde2 1]

The CIP spec defines this: The maximum size of the MR request and MR response packet items in the SendRRData command is 504 bytes (the maximum size on ControlNet), to ensure that a UCMM message can traverse all the links in a CIP network path. Devices may support the Large_Forward_Open service (see Volume 1, Chapter 3) to allow more efficient access to large application data sizes. and also: Connection Size The CIP connection size shall be no larger than 65511 bytes. NOTE: The Forward_Open request limits the connection size to 511 bytes; however, the optional Large_Forward_Open allows larger connection sizes. I know some devices support large forward open now, so this limit is dated and I would consider it product specific. Practically, a particular product likely implements some limit and that is what you would design for; however, other devices may have different behavior.

[Ballen22 0]

thanks guys

[daba 4]

Whatever the maximum bytes is quoted, It is much more convenient to think of the limit as 500. 500 bytes = 500 SINTs 500 bytes = 250 INTs 500 bytes = 125 DINTs 500 bytes = 125 REALs Sure, you can squeeze through a few more SINTs, and an extra INT or two, but stating the limit as 500 means you don't have to worry about data-type. My 2c