Re: Fast, reliable remote I/O in 2015

[<nick.rees@diamond.ac.uk>]

Hi Andrew,

The Ethercat option you referenced doesn't use a PLC - it directly connects to the hardware via a spare Ethernet port on a Linux system with an open source Ethercat driver and an EPICS driver we wrote. We typically run the Linux scanning task at 1 kHz and have typical latencies in rhe 1 ms range.

It probably matches your requirements pretty well and I heard of a number of sites who have tried it and reported success.

Cheers,

Nick Rees
Principal Software Engineer           Phone: +44 (0)1235-778430
Diamond Light Source                  Fax:   +44 (0)1235-446713


-------- Original message --------
From: "Gomella, Andrew (NIH/NHLBI) [F]"
Date:19/08/2015 22:08 (GMT+00:00)
To: tech-talk@aps.anl.gov
Subject: Fast, reliable remote I/O in 2015

Hi Everyone,

We are in need of some simple, yet fast remote I/O solutions in our lab. Namely we are looking digital inputs and outputs accessible as EPICS PVs for various fast synchronization tasks.

Our current solution has been to use National Instruments DAQ boards which has been relatively unreliable (if the computer shuts off, so does the DAQ; No EPICS support that I know of from any facility; Only well supported by NI with Windows use, their Linux drivers do not work for us) They seem much more suited to isolated measurement tasks, rather than as "always on" hardware controllers.

I imagine most people in the EPICS world would use PLC's for this, as I found in a Tech-Talk thread on essentially the same topic in 2009 : http://www.aps.anl.gov/epics/tech-talk/2009/msg01775.php or use VME crates (it seems there is a gradual trend away from using VME for simple I/O tasks)

We are looking for a simple, reliable solution that in the end results in simply having one record per digital in, one record per digital out. Ideally it would be very fast so we could act quickly on callbacks to input value changes ( <1ms would be ideal but 5ms is acceptable).
We would likely have must of our logic running elsewhere in Python, so we do not necessarily need ladder logic or other embedded software to execute logical tasks (besides maybe what action to take if a watchdog timer fails).

After searching tech-talk I compiled this short list of options, though I am curious if anyone else has better ideas for this specific application since 6 years has passed since some of these threads:

Koyo PLC's with softIOC using the modbus EPICS module
-Mark River's notes here that this allows direct access to memory registers so it can be used as a "simple I/O system" http://www.aps.anl.gov/epics/tech-talk/2009/msg01797.php

Yokogawa PLCs with embedded Linux
- as demonstrated by KEK here: http://www-linac.kek.jp/cont/epics/f3rp61/ and discussed here:  http://www.aps.anl.gov/epics/tech-talk/2009/msg01795.php
-Having embedded EPICS on a PLC seems like a very attractive idea.

WAGO or Beckhoff PLCs using EtherCAT as recently described here
-http://www.aps.anl.gov/epics/tech-talk/2014/msg01315.php

(Epics Brick seemed like a great idea when I first read about it then realized it was discontinued in 2008)

I have to note that it seems many PLCs digital inputs are not as fast as we would like (the entire Koyo Click series seems to only guarantee a max response of 10ms for off to on and on to off so that removes this option for us).

I'm sure a lot has changed in the past 6 years, and that I missed some options so any advice would be greatly appreciated.

Thanks in advance,

Andrew Gomella

Imaging Physics Lab, NHLBI, NIH


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