I understand. I'm working out of town and just came back for the weekend.
For me this is a good time to be distracted with too much work. It helps me ignore the wind gusts to 30 mph, the snow, and the temps headed to 8 degrees F for tonight. Except that I need to drive a long ways in this crap tomorrow morning. Dave On 1/10/2016 3:59 PM, John Thornton wrote: > Still trying to digest this in between working... > > On 1/10/2016 7:48 AM, Bertho Stultiens wrote: >> On 01/10/2016 12:44 PM, John Thornton wrote: >>> Well none of the suggestions about star grounding did any good. In >>> fact they made the problem worse than before, so bad I could not even >>> get 2.6 to run. >> That may suggest a couple of things being wrong simultaneously: >> 1) you may still have loops in the 0VDC which are crossing power >> domains. These /are/ hard to detect with many devices connected together; >> 2) there are impedance mismatches in the signal wiring (this one often >> works together with 1); >> 3) shields may pick up noise they should reject. >> >> The question is how to reduce the practical problems, not how to >> eliminate all theoretical problems. That is the hard part. >> >> >>> Off this list I was sent the AB servo best wiring practices pdf >>> which explained how to reduce noise in a servo drive enclosure. I >>> followed the advise as much as possible and after removing all my ground >>> wire antennas I was back running with an occasional sserial error. The >>> short explanation is to terminate the shields to a ground plane as close >>> as possible to the point where you take the shield/drain wire from the >>> cable. I still have a cable bundle running to my switch box that is >>> parallel wires and I might replace that with some twisted pairs. Photos >>> at 11. >> You refer to this document? >> http://literature.rockwellautomation.com/idc/groups/literature/documents/at/motion-at004_-en-p.pdf >> >> That doc explains very well how to separate into different power zones. >> Also, it makes a good case of shielding and how to do it properly. BTW, >> note that it uses protective ground as shield potential and _not_ 0VDC. >> That is also why the argument of two-sided termination of the shield is >> appropriate (see page 21). >> >> But all this does not save you from impedance problems. The sserial >> errors are interesting because it is probably something you actually can >> measure with an oscilloscope. >> >> You should be able to see the signal integrity and whether you have >> reflections of the signal at either end of the cable. >> >> The sserial connection has a great potential for a loop. See attached >> image for illustration. The image has a shielded connection between two >> devices, which is good. However, the 0VDC connection (aka GND), which is >> embedded in the shielded connection is shorted outside the cable via a >> protective ground connection through the PSUs. This means that part of >> the signal return may circumvent the constraints of the signal cable and >> that creates an unbalanced path. Effectively, it means that the shield >> is now part of the cable's impedance, and that is guaranteed to be off >> by a factor. >> >> You can check for this scenario by disconnecting the sserial connector >> at one side and measure 0VDC vs protective ground on the connector on >> the other side (and the other way around). You are in trouble when >> measurements on both sides indicate a connection between 0VDC and >> protective ground. >> >> If you only have one PSU for both devices, then you are potentially also >> in trouble. The PSU will then act as the loop facilitator (through two >> distinct 0VDC connections). However, the effects are more subtle and >> depend on coupling effects. >> >> The rule for a shielded cable is that all energy must be contained >> within the cable to be effective. In other words, the sum of all >> currents in the cable's wires must be zero. Bypassing some of it may >> cause impedance and therefore signal integrity problems. >> >> >> >> ------------------------------------------------------------------------------ >> Site24x7 APM Insight: Get Deep Visibility into Application Performance >> APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month >> Monitor end-to-end web transactions and take corrective actions now >> Troubleshoot faster and improve end-user experience. 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