Yikes! Look out for that Chasm!

Dec. 3, 2007
The best practice by far is to choose a main instrument vendor who is accountable for the integration of all field devices.
By John Rezabek

Did your foray into fieldbus land you in a chasm? October’s issue of Control featured an article by Dan Hebert, where he managed to find a lot of people who apparently have, or who fear they will end up in such a deep hole. Based on my experience, I’m curious how all these folks landed there.

When our site undertook its first fieldbus project eight years ago, we had some trepidation about what chasms might lay in our path. But our goal was to be “like the background music in a movie–if it’s really good you don’t even know it’s there.”

Without any superhuman effort or acts of sheer genius, we achieved that goal, even while declining additional (free) support from our main instrument vendor (MIV), in our case Fisher-Rosemount, who was eager to assist and highly invested in our success. Pondering the relatively easy path we followed to full digital integration of field devices, what crevasses were we were lucky(?) to have bypassed?

Crevasse Route 1. Install an ISA Show in your plant.  “Freedom to Choose”–that’s a trademark of fieldbus. But those who have chosen to install one device from every vendor who’s bought them donuts in the last decade will have a tougher path than those who standardize on relatively few. The best practice by far has been the MIV route mentioned above: Make one vendor accountable for the integration of all the field devices, even those that they don’t make. That way, you’re not in the chasm in which one of Dan’s contacts found himself, struggling to “find the guilty” when something didn’t work.

Your first project, at least, will benefit greatly from having an MIV who is positioned to leverage the resources of host and device suppliers alike. This entity is, more often than not, your host supplier. I would save the independent integrator for my second fieldbus job.

Crevasse Route 2. Recruit electricians from the video store. I’ve heard a number of travel-weary troubleshooters describe the tangled mess they’ve found in a junction box. Instrument terminations−fieldbus or otherwise−are not a chore that should be left to the journeyman still recovering from a late night watching football at the local pub. Wrong terminations, loose terminations, sloppy crimp jobs and cables pulled in with a forklift can make what should be simple into a complicated mess.

Shield termination creativity is another source of woe. Dave Smith of Yokogawa tells a tale of some chasm-dwellers who had landed the twisted-pair shield at both the system and the device. Dave fixed the network so that the shield was only landed at one end. Clear communications commenced, and the sun shone in the chasm as it hadn’t in the two years before. Unless you’re in a very compact and uniformly grounded facility, multiple shield grounds could also doom you to months or years in the shadows.

Crevasse Route 3. Railroad your technology choices on the end users. One of the surest paths to a failed project is to implement it in a way that doesn’t respect the domain of local operations and maintenance. Whether through slide shows, baseball caps or training junkets to warmer climates, every new technology project−not just fieldbus−benefits enormously from gaining the support and buy-in of local stakeholders.

Where an entrenched culture already exists, choices that seem to be imposed arbitrarily will get a cool reception, if not outright sabotage. Take the time to sell your “clients”−whether internal or external−on the new technology you’re about to rain on their world. Otherwise, you may create a chasm that you’re no longer welcome to visit.

Crevasse Route 4: Make it more complicated than it is. One of the folks Dan interviewed was busy addressing his or her FF devices. I haven’t assigned a single address in eight years. It’s curious that people who don’t flinch at Modbus with all of its parity, stop bits, hand-shaking and data-type traps find fieldbus complicated. Only the most primitive fieldbus implementations still make you assign addresses, and most modern ones are approaching plug-and-play. I’ve had more issues getting my “XP Home” PC to network with my “XP Professional” one.

There may be an abundance of chasms, but common-sense choices used for most of our engineering endeavors will steer you far from most of them. If you feel like you’ve been left in one, I’d be interested to hear about it.