Dear friend and reader,
When it's 107 degrees in the shade...
....customers get downright cranky without their air conditioning.
Today, the story of a trip I took a few months back to help solve some RS-485 problems for a customer. The near 100 degree temperatures here in Illinois reminded me of the similar temps out in Arizona when I visited Salt River Project ("SRP") last spring.
SRP supplies power to one of the fastest growing counties in the US. Serious expansion of the electrical grid. Those engineers have enough to do without having to worry about communications glitches in their SCADA equipment. For SRP, not only were those glitches causing erroneous trouble reports, but they threatened to mask the reporting of real problems in the substation.
So the ongoing data errors reported by SRP’s multi-vendor SCADA systems deployed in their substations had been the source of stress and concern for years.
Not helping was the fact that virtually every vendor in the system had implemented different circuits and recommendations on how to wire, ground, bias and terminate their RS-485 ports.
Sound familiar?
I’m sure that all these vendors make great controls, relays, breakers and switches. Probably some of the sharpest engineers out there. But that doesn't mean that they know squat about RS-485. Who can blame them? Serial communications isn’t their business. And I’d wager that their most battle-scarred seasoned engineer wasn’t the one assigned to the com port portion of the project . That job was probably handed to the new guy, still a bit wet behind the ears, to cut and paste some existing code and schematics together from prior products to get it working.
It’s no wonder then that a customer like SRP had been having difficulty for years getting the communications working reliably. One of their tech’s contacted me and explained their woes, so I waited for a good hot day, jumped on a plane and visited their shop.
I liked these guys and gals right off the bat. They had already read most every white paper and tech note that I had published on RS-485. They had already studied grounding, shielding, termination and biasing of their systems.
Every one of them even managed to stay awake while I sermonized at the white board as we reviewed all the basics of RS-485.
But anyone can draw squiggly lines on the white board. The proof is in the pudding.
We did some testing in their lab to validate that the whiteboard concepts could be measured with a meter and scope. So far so good. Quick break for some of the best Mexican food in Arizona (but sadly had to pass on the Margaritas) and we headed out to a wayward substation.
230,000 Volts:
Do NOT Touch!
If every shelf in a fine-china shop has a “do not touch” sticker on it, one would think that you’d see a lot more warning labels inside a 230,000 volt substation. I wasn’t even the slightest bit offended when one of the SRP guys reminded me not to lean on the rack that was 8 inches behind me as I poked at the RS-485 signals with a scope.
At the substation, we took a quick peek at the signal levels and made some minor tweaks to the termination and biasing. Making those tweaks was easier than ever since SRP had adopted our new 485DRCi converter with switchable biasing.
(This baby is triple isolated, too - see http://www.bb-elec.com/product.asp?sku=485DRCI May help you eliminate an extra power supply too.)
But looking at the data on the scope revealed that there were some misbehaving relay slave nodes on this multidrop RS-485 system. Some relays weren’t always replying to their polls. Well, they were replying, but only after some seemingly random delay and causing bus contention.
Problem identified!
Some diligent note-taking and follow-up calls to the relay manufacture led to a software update and the problem was solved.
Have a problem with your RS-485 communications? Don’t let it fester – we can help!
For starters, take a look at all the great info on http://www.bb-elec.com/technical_library.asp. The answer to your problem is there. But if it’s not jumping out at you, give us a call. We can solve your RS-485 problems too.
See my whole family of 485 converters at:
http://www.bb-elec.com/SubCategory.asp?SubCategoryId=9 (Including units for fiber optic cables, Allen-Bradley PLC's, and current loops)
And hey, maybe next trip, we can get to the Margaritas....
Happy Connections,
Mike Fahrion
support@bb-elec.com
(800) 346 – 3119
P.S. If you haven’t gotten your new catalog yet, issue 46, I’ve got bad news for you - it ain’t coming. Catalogs have gone out and our phones are ringing off the hook. So if your copy has gone AWOL or you never received one, please visit http://www.bb-elec.com right now and click the catalog request link
|
