PT100's on a bus system - but which bus?

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Thread Starter

Mark

I have been looking at ASi for analogue kit but did not get a very convincing response from chaps here, so what else is there? I want to connect three wire PT100's to a local box which is connected to a network with other boxes back to a PLC or SCADA system via a bus system. The installation and configuration needs to be minimal. I would like to run only a power cable and the comms cable to these 'boxes' which would have to be IP65 or better and work -10degC to 45degC. Location to be on top of a low temp cold store and the probes can be 100mtrs apart at the extreme corners. The master PLC is 100 mtrs away from the store. I know there is Profibus and FF and the like, but associated PLC manufacturers have rack/slot orientated kit which is not robust enough (also not particularly cheap)unless its mounted in an heated enclosure. I'm trying to justify using a bus system instead of hardwiring to help improve installation speed, and reduce wiring errors.
 
> ....... snip > > I want to connect three wire PT100's to a local box which is connected to > a network with other boxes back to a PLC or SCADA system via a bus system. Have you looked into the GE Multilin RRTD (Remote RTD Module). This unit can accept up to 12 3-wire RTDs of various types and communicates back to a host via Modbus. The standsrd unit is hard wired using RS-485 but Fibre Optic is available as an option. A Profibus port can also be obtained. Their web site is: http://www.geindustrial/pm Ian Milne Upside Engineering Ltd. Calgary, Canada
 
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IP OMER BIN ABDUL AZIZ

If it is possible to extend the probe cables, then I guess you can extend all the PT100 cables to the central PLC (the PT100 maximum distance is limited by the total resistance of the cable). How many total probes do you have, and what is the PT count per probe? I am also working on a similar project. The probes are used for wheat silos. The original system came from some italian company that has ceased to exist now! The system consists of a central FIU and field units. The field units are communicating with the FIU over a single bus. In my case, accuracy and quick switching time is not a big issue, so I am working on a minimum PLC I/O system, maximising the use of Relays used as make-shift multiplexers. Best Regards! Omer
 
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Kai Atle Myrvang

Hi! suggest you have a look at PEP Modular Computers Smart2. It is a small modular computer that can be equipped with PROFIBUS-DP, CAN or Ethernet. Some 10s of PT100 can be connected to one unit. (40-50 I think). Reason for suggesting the unit is that the price pr. channel gets reasonable and it can be delivered for -40 till +85 deg.C. On the down-side ... it is NOT IP6x. But that can be solved with a small cabinet. For many installations an acceptable compromise. Have a nice week-end. Kai Atle Myrvang AD Elektronikk AS Oslo - Norway
 
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Sztrancsik Csaba

I think you would like to spare wiring only on the Pt-100 measurement units. Did you look after HART ? Possibly not too cheap, but has some added functionality. Regards, Csaba AtySoft Ltd., * TEL: (+36 1) 316 3251 * FAX: (+36 1) 212 0250
 
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rajesh kunhi kambrath

mark I think MTL is having somewhat similar system what you are looking for. Basically Multiplexing/Demultiplexing the RTD/Thermocouple signals. But I Am not sure wheather multidropping is possible in this case or Not. Please verify with MTL
 
> In my case, accuracy and quick switching time is not a big issue, so I am working on a minimum PLC I/O system, maximising the use of Relays used as make-shift multiplexers. A word of caution there. I have seen relay switching of PT100 sensors some years back, and my firm was brought in to replace the switching with hardwiring because the contacts only need a small ingress of dust or dirt and the temperature readings drift about because of the contact resistance. You only need a small resistance to affect readings.
 
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Vitor Finkel

To switch PT100s (RTDs), use reed switches ( or reed relays ) they are probably the only acceptable cost/benefit answer to good contact with the low voltages and low currents involved. Vitor Finkel [email protected] P.O. Box 16061 Tel (+55) 21 285-5641 22221.971 Rio de Janeiro Brazil Fax (+55) 21 205-3339
 
Try I2C. It's very simple and a very save busstructure! For more information, look at the philips site.

 
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