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AI to AO Direct Hardwire

AI to AO Direct Hardwire

AI to AO Direct Hardwire

An analog output signal generated by SIEMENS DCS needs to transfer a package unit PLC (also by SIEMENS) through an analog input channel as depicted below. Question is, will work or an isolator will be need?

RE: AI to AO Direct Hardwire

Since you have both a PC system and a DCS, I will assume that each system with have it's own 24Vdc power supply. Unless either the DCS OP or the PLC IP card is isolated, I would install an isolator between the two devices.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: AI to AO Direct Hardwire

To irstuff
Because both cards generat loop powered signals.
They are both in a control room.

RE: AI to AO Direct Hardwire

This reminded me of such a system

It is used to debug the control algorithms on the PLC, PC plays the role of a plant.

RE: AI to AO Direct Hardwire

I still don't see a problem. The Siemens' modules were intended to interoperate, since their HART configurations are ostensibly the same. The only reason for using an isolator might be a hazardous environment, or they were on some sort of completely different ground and AC power system.

TTFN (ta ta for now)
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RE: AI to AO Direct Hardwire

IRstuff, among the beginning engineers there are a lot of terrible stories about how analog inputs die. Simply AzArsha is not sure.

RE: AI to AO Direct Hardwire

You haven't supplied enough information. For example are the analog outputs isolated from one another (no common wire)? Also are the AI on the package system isolated? If the answer is no to both, then you will need isolators. In general you cannot connect two non isolated devices.

RE: AI to AO Direct Hardwire

Some of the high channel count I/O cards are horribly expensive to replace if a channel gets damaged. Isolators are cheap protection. Why wouldn't you?

RE: AI to AO Direct Hardwire

I'll have to admit I'm not familiar with the specifics of DCS architecture, but surely it'd be better to push the data over a communications link rather than HART links per signal?

I know its off topic, but it seems unusual that this arrangement would still be required, unless this is an existing plant that doesn't have a higher level communications ability.

EDMS Australia

RE: AI to AO Direct Hardwire

1. Are you intending to use the HART signal or is the 4-20mA signal the 'data'?

If the process needs whatever's on HART and you use isolators, then you need HART transparent isolators. Most isolators strip off the HART and just pass the 4-20mA through, so the output side of the isolator has no HART signal. In fact, some isolators have a low pass filter on the input side that will strip off HART, too. HART enabled/transparent isolators are clearly marked and priced accordingly.

2. If the boxes are in the same building, you might get away without isolators. Can you connect the points and try and see whether the common mode/ground loops offset or drive the signals off scale? No offset, no problem.

3. Both racks are Siemens. Why not Profibus?

RE: AI to AO Direct Hardwire

This topic suddenly became popular!

Quote (Some of the high channel count I/O cards are horribly expensive to replace if a channel gets damaged.)

Yes, it's really annoying, especially if such equipment is used for training purposes ... Serious guys offer a cheap kit DAQ- Motherboard + known modules and microcircuits. In their boring video lessons and in complete silence, they show examples of the use of this junk.

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