>>Message imported from previous forum - Category:Realflo Electronic Flow Measurement<<
User: mchartrand, originally posted: 2018-10-17 18:33:00 Id:74
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.
I have 3 SCADASense 4102 MVT's connected in parrallel RS485 communications to a SCADAPack 334. The issue I am running into is when all 3 are powered up I can't communicate to any of them using real flo, however if I power down any one of them I can communicate to the other two with out any issues. The MVT's are addressed 2, 3 and 4. When all three are powered up two of the displays show proper information and the third says reading unavailable. Has anybody run into this issue before?_**
Are you also sharing power between all these MVTs?
No, they are all powered separately._**
Is it the same one that you've always powered off?
Darrin, how are you wiring these units from an RS 485 perspective?
This post has not been updated in a while and I imagine the problem has been solved. But to solve the riddle for others to benefit: RS-485 has a particular wiring method that should be followed for optimal performance. It is a multi-drop type system and should not use a star wiring method (which is like most ethernet networks).
Star topology has a central point (your SP334 and panel) and branches out to each MVT. This causes problems with RS-485 because the different length of the branches creates problems for the digital signal. You can end up with reflections of the digital pulses or other strangeness.
Best way to wire is to run your comm cable out to the first MVT then daisy chain out to the second MVT and then finally to the third ending the run with termination resistors (optional in some cases).
Do a search in google for "RS-485 wiring" and you should come across some decent sites with information about proper wiring of your communications cables.
I have not been on the site for some time and completely forgot about this thread. For future reference always check your wring when the multidrop connection is done on the MVS it self. This instance was on a new construction installation where 16 gauge wire was used instead of 20. When i started checking the individual units i found that the installers did a poor job of terminating the wiring (due to the heavier wire used) and there were strands touching between the two wires basically causing a short and messing up the communications. After re-terminating all connections at each MVS the problem was resolved.
Lesson Learned: Don't trust other peoples work as being correct/complete._**