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Old 08-22-2012, 09:50 PM   #1
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Default rs-485 siemens problems

To start off with I have not ever worked on this system or any other similar system so please hurl the insults. I am going tomorrow to give it the old college try. I have a customer that has a siemens control system (don't know the type but I stab that it's 10 years old based on other things) that's losing communications then going into a shutdown sequence on the vfd's for the cooling fans on the hvac system. They said it's RS-485 on a serial cable. They are thinking it could be a grounding issue due to the dry weather and the ground rods losing their connection to the earth. Also they are wanting me to verify that the serial cable is properly grounded.....i don't know how to do this other than a continuity check or unhooking both ends and megging the cable itself. I realize no conclusions can be drawn but maybe I can get a starting point.
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Old 08-22-2012, 11:33 PM   #2
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Basics of troubleshooting RS-485

http://pdf.directindustry.com/pdf/b-...059-94167.html

Usually when I had communications trouble it was a bad 485 transmitter / receiver / converter to rs 232 or Ethernet.

Check terminating resistors if any. They are only 90 to 120 ohms and could act like a fuse if any surges hit.
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Old 08-23-2012, 12:26 AM   #3
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If it is a Siemens INDUSTRIAL control system, then it is likely Profibus, which uses the same hardware as RS485, but is different. 9 times out of 10 with that, it's a bad connector, especially if they are not factory made.

If it is a Siemens Building Technology (HVAC) control system, they are totally different, only distant relations to Siemens Industrial, so be aware of that. Siemens HVAC is the old Landis & Gyr, combined with the old Staeffa Control Systems. Whatever they do/did for HVAC serial communications is going to be something different than what the other Siemens Automation people do, no telling what kind of stuff you will run into there. They tended to run things a little loosey-goosey over there when I worked for Siemens. A lot of the comm lines were done with bell wire and such, little or no shielding etc. etc. I guess they could get away with it in office environments.
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Old 08-23-2012, 01:02 AM   #4
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Is there a particular procedure for testing the connections? I believe it would be the Profibus.
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Old 08-23-2012, 05:16 PM   #5
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Ok couple of things, its likely going to be wired with 18/2 or 18/3 shielded cable communication cable.

Make sure the shield/ drain wire is spliced through at each termination. The loop should be in a daisy chain and should only have 2 cables per termination. Taps are bad.

End of line resistors depends on the communication protocol and may also exist at the origination of the trunk. The shield/drain wire should only be spliced through each controller and only be grounded at one end (usually at the origination of the trunk).

Theres most likely an A, B, Ref and shield. Do not ground the reference wire. I would advise against megging the cable. I doubt your problem will be related to ground rods. Usually when a ground loop exists on communications you can get away with a "floating" ground. Systems such as LonWorks don't even use the reference wire. Many times startup techs will remove my reference wire as it could cause more harm than good.


Your issue could be a failing controller as well. A controls tech can tie the communication trunk into a USB- RS485 converter and capture the data on the trunk and analyze it to see if any one controller is putting garbage or noise on the loop.

Sometimes solid send / receive indicators will help you identify a controller that is locked up...
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Old 08-23-2012, 05:20 PM   #6
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You should read DC voltage between REF to A and REF to B. It will be low for example 2.5vdc but again depends on the protocol and controllers in place.
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Old 08-23-2012, 06:46 PM   #7
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Quote:
Originally Posted by JRaef View Post
If it is a Siemens INDUSTRIAL control system, then it is likely Profibus, which uses the same hardware as RS485, but is different. 9 times out of 10 with that, it's a bad connector, especially if they are not factory made.

If it is a Siemens Building Technology (HVAC) control system, they are totally different, only distant relations to Siemens Industrial, so be aware of that. Siemens HVAC is the old Landis & Gyr, combined with the old Staeffa Control Systems. Whatever they do/did for HVAC serial communications is going to be something different than what the other Siemens Automation people do, no telling what kind of stuff you will run into there. They tended to run things a little loosey-goosey over there when I worked for Siemens. A lot of the comm lines were done with bell wire and such, little or no shielding etc. etc. I guess they could get away with it in office environments.
Do you remember the names of that system before it was Landis & Gyr?
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Old 08-23-2012, 07:07 PM   #8
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If its a VAV that's hung up, it's just lost its mind.
Unplug it and power it down. (just the VAV).
When you plug it back in the panel will reload the program.
If the panel has no computer front end, you will need the password and cable to get it. Don't power down the panel, you will wipe it clean.
Dial in if you can using hyper terminal if it's an older panel.

Their token ring has a habit of getting lost.
The program they use is PPCL. It's a take off of C+ & basic.
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Old 08-23-2012, 09:19 PM   #9
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rs-485 siemens problems-image-327962264.jpg

Thank you all for the help the factory rep is coming tomorrow. I basically passed and will come in to learn from their rep.
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Old 08-23-2012, 09:43 PM   #10
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Quote:
Originally Posted by garfield

Thank you all for the help the factory rep is coming tomorrow. I basically passed and will come in to learn from their rep.
We have hundreds of them here at work. The VaV just lost its mind or the panel dumped its program. Everything else is just a snap in module.
As for their catalog and price list for books.
It's pricey but worth it if you do alot of seimens.
I've been thru their classes down in Buffalo grove il.
Nice enough place and people. They can be very customer orientated. But they strive for service contracts. It's a good gig cause no one knows how they program and everything is proprietary.
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Old 08-23-2012, 11:15 PM   #11
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Quote:
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Do you remember the names of that system before it was Landis & Gyr?
Hmmm... MCC Powers?
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Old 08-24-2012, 01:53 AM   #12
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Quote:
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Hmmm... MCC Powers?
Landis & Gyr bought Powers in 1987. Siemens bought them in 1999.
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Old 08-24-2012, 07:14 AM   #13
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Quote:
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Hmmm... MCC Powers?
Yeah it was POWERS
before that it was MCC POWERS
and before that it was MARK CONTROLS CORPORATION

1984 was my first exposure to the brown cabinets and the cassette programming of panels.
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