What standard describe AI and DI signals

[leomiranda]

I need to know what standard describe in detail transmition of analogical and digital signals. I need to prove (based on standards) that it's possible transmit analogical and digital signals in the same multicable without problems.
Obrigado! thankyou!

 

[itsmoked]

You won't find that in standards I don't think. And you may well have problems with co-routed digital and analog signals. It's not a great layout.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I think that you first should model your transmission paths. Use transmission line models, mutual coupling, resistance and such. Then check to see what your signals look like whan they have travelled down the cable. If the signals look bad - then don't do it. If they look OK, then compare to the limits given for your signal category. There are quite a few accepted de facto standards like the TTL lo/hi and gnd bounce, the PLC lo/hi levels, the RS232 0/1 levels and lots more. You need to be more specific.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[leomiranda]

Thank you! I will be more specific.
I work with instrumentation projects, sometimes the client is the Petrobras. There is one Petrobras standard that says “Instrument cables have to be connected in junction boxes, segregated by signal and by level tension” (Os cabos provenientes dos instrumentos de campo deverão convergir para caixas de junção, segregadas por tipo de sinal e nível de tensão). Only this!
But I have one situation in my hands where the instrument cables (analogical and digital) are connected in the same junction box. The manufacturer provided your skid with the instrument cables connected as mentioned above. In the skid there are 6 pumps. Each pump sends temperature and pressure signals (analogical 4~20mA) to your junction box. But in each junction box there are 3 digital signals too. They are: start, stop and Local/Remote for the pump. All signals leave the junction box in the same multicable. But, in my opinion, this is a special case, because the digital signals will never create interference on the analogical signals.
If the pump is stop, to start or to select L/R won’t create interference, because the pump is off. If the pump is on, to stop or to select L/R could create one interference, but not significant. If I press stop, the interference doesn’t matter, because the pump will stop and the analogical signals will stop to be send. Select L/R isn’t a command that will be done constantly, so it wouldn’t create a significant interference on the analogical signal.
But Petrobras want to change the connection, based in your standard.
I’d like to know which international standad shows it in detail.
All signals are 24Vcc. The multicable has individual and overall shield. All pairs are twisted.

Tky

 

[Skogsgurra]

I think that you should look more to what is accepted practice in your industry and part of the world. If a manufacturer that produces equipment for your industry (refinery?) has done this over the years and if nothing has been changed lately, standarwise. Then you probably do not need to bother.

In general, I would not be too concerned if you are sending 24 Vdc PLC signals and 4-20 mA signals in one cable. They are rather low level, have low high-frequency contents and are usually BW limited at the inputs.

The individual shields makes it even less critical.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JLSeagull]

Some DCS site planning manuals show 24 Vdc / 4-20 mAdc or high level analog inputs with 24 Vdc digital signals. Thermocouples are low level signals and may be excluded from the digital signals. Forget about ISA 50.00.01 "Compatibility of Analyg Signals for Electronic Industrial Process Instruments". It is an applicable standard for analog signals that lacks any mention of the digital signals. I don't see anything helpful in the intrinsic safety standards.