analog input cards

Taggart

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I am haveing trouble understanding the difference between the 4-wire analog card of the siemens series plc and the 2-wire analog cards. From what i see the 4-wire is set up as a passive system, but am not sure if the 2-wire would be considered passive or active. Just checking to see if anyone could shed some light on this for me.
 
From my recollection, the 4-wire system measures the voltage at the device and effectively compensates for the volt drop in the wires from the device to the analogue card. A 2-wire measurement does not.
 
4-wire : your instrument or device is supplied with power ( 24V and 0V) and your PLC uses the second two wires for the analog value ( + - )
2-wire : to me its actualy 3 wires because you NEED to use the ground or earth or analog ground ( whatever they want to call it)
The First wire from the PLC supplies 24v to your instrument and uses ground as zero volt .Your second wire is your actual milli-amp feedback ( the + ) and again the PLC card uses its ground for the analog '-'.Your PLC and device must beproperly grounded . Then there is trhe question of your shielded cable where the shield is only to ground on the one side.( to prvent current loops ... in case of bad grounding)

Hope it helps

Eric
 
Tag,

WOuld be easier to give you an answer if we knew if you you mean an analog input or analog output module, I will assume analog input module because thier really isnt a 4 wire analog output. And would also be good to know if you are refereing to a voltage or a milliamp signals.

Assuming analog inputs:
- The four wire input module would provide DC + and - on two terminals and would use the other two terminals for the signal.
- THe two wire would just be for the signal, and if the instrument (transmitter or whatever)needed DC power that would have to be provided from elsewhere, typically have adjacent terminals on a terminal strip somewhere so all leads from the cable can be all togeter next to each other. Or if the instrument/transmitter needed AC power that would typically be from elsewhere and not in same cable.

DAVE
 
Well this topic certaintly has gotten mangled with contradictory info. It's more of an instrumentation topic than a PLC one. Let me take a shot at clearing it up...

Analog signal transmitters (pressure, temperature, flow, level, etc.) come in two basic types: 2-wire and 4-wire.

The significant difference is that a 4-wire transmitter is powered. That is, it has terminals for power connection on the transmitter. It can be 120vac or 24vdc. The signal from a 4-wire transmitter is powered from the transmitter. No loop power supply is required.

On the other hand, a 2-wire transmitter does not have a separate power connection. It usually only has two terminals (+ & -). It derives any power it needs from the loop power. Therefore a loop power supply is essential. Power is not usually supplied from the PLC input card. (Not saying it can't be, just that the I/O I deal with doesn't.)

It is possible to get fooled. For example, Micromotion does have a version of it's series 2700 flow transmitter that requires a 24dvc power connection but the signals are wired as 2-wire devices and require loop power. But this is pretty rare.

Note: L D[AR2,P#0.0] seems to be talking about 2 and 4 wire RTDs.

The 3-wire devices aikona is talking about can be considered to be 4-wire with the power supply com and the signal com shared. Usually found on cheaper devices.

Hopefully this helps rather than adds to the confusion...
 
Most (NOT ALL) Siemens ET200 type 2 wire capable input modules have internal supplies and you don't need to provide external +24VDC supply for loop power.

Nick
 
2 wire analog transmitters are powered by Analog I/P of the input card.
4 wire transmitters are given seperate supply on two wires and the other two wires carry measured signal to the analog input card. Usually the - of the seperate power supply and the analog card - are common.
 
manmeetvirdi said:
Q)How many RTD(2 wire or 4 wire) you can connect to 8-channel analog input card?

A)Four

RTD : could it mean Run Time Device or Remote Temperature Device or

any case RTD are allways 4 wire or am I wrong

Eric
 
manmeetvirdi said:
Q)How many RTD(2 wire or 4 wire) you can connect to 8-channel analog input card?

A)Four

Do not get this wrong: 2 wire and 4 wire are indications of the device, NOT of how many wires you need to connect to the AI-Card.

In either case you should connect 2 wires max to the AI-Card. A 8 channel AI-card will be able to handle 8 analog inputs.
 
aikona said:
2-wire : to me its actualy 3 wires because you NEED to use the ground or earth or analog ground ( whatever they want to call it).
The First wire from the PLC supplies 24v to your instrument and uses ground as zero volt .Your second wire is your actual milli-amp feedback ( the + ) and again the PLC card uses its ground for the analog '-'.Your PLC and device must be properly grounded . Then there is the question of your shielded cable where the shield is only to ground on the one side.( to prvent current loops ... in case of bad grounding)

Can you provide a rough sketch for 2-wire??

Do you mean that transmitter's ground and Analog Input module ground are common?

We normally ground shield at one end only because if we ground both ends there will be a current loop with shield and ground as conductors and this results to interfere with the signal which is flowing in the twisted pair....Am I correct?
 

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