Experimental Physics and
Industrial Control System

"Kasemir, Kay" <[email protected]> · Wed, 06 Apr 2011 08:31:22 -0400

Hi:

Steve Schuh already quoted the readme.
There's also the changes.txt file,
http://epics.hg.sourceforge.net/hgweb/epics/ether_ip/file/tip/changes.txt,
that sais more about what can happen when your arrays are too big.
Look for "Buffer too small, partial data only".

The exact limit is still fuzzy to me.
It's around 500 bytes, according to an email I received from a Rockwell
engineer:

---
Packet length limitations of CIP data:
I/O: 500 bytes for input data, 496 bytes for output data. They differ in
length because the output tag does not include the four bytes of real-time
header.
Explicit messaging, connected: ~486 bytes sending/receiving
Explicit messaging, unconnected: 502 bytes. When sending data unconnected,
the route path size must be subtracted (minimum1 word), since EtherNet/IP
includes the IP address in ASCII format.
---

Sooo, you can make your REAL arrays bigger than 32 or the 40 suggested in
the readme. One REAL uses 4 bytes. REAL[40] is just 160 bytes, well below
the packet length limit of ~500.
REAL[40] was a suggestion because that way several of those REAL[40] arrays
can be transferred within one packet, allowing the driver more flexibility
in combining transfers.
I would advice against going as high as you can based on trying "one more"
until you get the "Buffer too small" error, because the exact size limit
seems to change with the IP address, firmware version, no clue what else.

Thanks,
Kay

On 4/6/11 24:11 , "Schuh, Stephen" <[email protected]> wrote:

> If you are talking about PLC data type REAL, the ether_ip readme advises a
> maximum array size of 40. For BOOL arrays the max recommended size is 350.
> 
> Excerpt from the readme:
> "Arrays should be one-directional: Use separate "EPICS to PLC" and "PLC
> to EPICS" arrays. Because of PLC buffer limitations, the array size is
> unfortunately limited to about BOOL[350] and REAL[40]. While you can
> define bigger arrays, those cannot be transferred over the network
> with EtherIP. Consequently you might end up with several transfer arrays."
> (See 
> http://epics.hg.sourceforge.net/hgweb/epics/ether_ip/file/tip/ether_ipApp/doc/
> readme.txt)
> 
> -Stephen
> ________________________________________
> From: [email protected] [[email protected]] On Behalf
> Of Stephen Lewis [[email protected]]
> Sent: Tuesday, April 05, 2011 3:53 PM
> To: [email protected]
> Cc: Stephen Schuh; Maida, Ricardo E.
> Subject: Ether/IP array limits
> 
> We are using ether-ip-R2-5 with Allen-Bradley ControlLogix PLCs and version 15
> of the AB (Rockwell) ladder logic.  Our previous engineer limited the arrays
> that are used to block transmit and receive variables to and from EPICS to 32
> elements.  Is there a limit or can they safely be expanded to 64 elements?
> The soft IOCs are hosted on RedHat Linux 4, 32-bit, built against base 3.14.9.
> 
> --Steve
> 
> Stephen Lewis
> [email protected]<mailto:[email protected]>
> 
> Linac Coherent Light Source
> SLAC National Accelerator Lab
> 2575 Sand Hill Rd | M/S-10
> Menlo Park, CA 94025
> 
> 
> 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System