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2-wire vs 4-wire
How to connect two 2-wire transmitters in series? Also they need to be connected to PLC.
By Rajasekhar Panda on 5 August, 2006 - 10:22 pm

I am new to this area...I have known that we can connect 2-wire transmitters in series upto 15nos.. how to connect them (an electrical diagram help me most)? What is the difference between the 2-wire vs 4-wire and when they are used at what condition?

It is not uncommon to connect many 2 wire transmitters to one large (usually 24Vdc) power supply. All transmitters would be connected in parallel, not series.

The transmitter vendor's documentation will have a diagram to show how to connect the transmitter in a circuit, but there's a crude diagram below.

Power supply = P/S
transmitter = xmtr
receiver (controller/indicator/recorder) = rcvr
wire = -----

--- (-) P/S + --- + xmtr (-) --- + rcvr (-) --

It is not feasible to connect more than one 2-wire transmitter in series.

It is feasible to connect more than one receiver device, like a controller input, indicator or recorder input in series, if the power supply voltage is adequate to drive the current through the combined resistances of the receivers and the wiring.


By Steve YATES on 9 August, 2006 - 10:52 am

Are these transmitters HART or Fieldbus devices by any chance? These can connected in parallel.

MTL instruments

By Robert D. Wagner on 10 August, 2006 - 5:39 pm

Four wire process transmitters/control elements are typically powered by a separate source supply. Therefore, 2 wires are for supply power and 2 wires are used for measurement signal (voltage or current). Because of the variations in the power for these devices, isolated inputs/outputs are typically used. Two wire transmitters/control elements on the other hand are powered by the same two wires which carry the measurement/driving signal. In some cases these transmitters are powered directly by the device which is reading the measurement or controlling the output action. In other cases a separate power supply is wired in series with the signal wires and the device reading/driving the signal. In this case a common supply can be used to power several loops.

Best Regards,
Robert D. Wagner, P.E.

Dear sir i need the help. why i choose 2 wire in place off 4 wire and where? When i could select the 2 wire or 4 wire?

By Robert D. Wagner on 18 August, 2006 - 11:33 pm

Two wire devices are typically preferred when available. They eliminate the need for separate power runs and the associated power distribution, circuit protection etc. to the device. As stated previously, they also often allow the use of a single power supply to power several loops, single-ended connection to input cards, and typically more inputs per card, decreasing the cost per point. Some devices, typically analyzers etc., may only be available as four wire devices, so you don't get a choice. A 4-wire device may actually need to be located remote from the process, or additional precautions taken depending
on the area classification of the process area, if the device is not available suitable for that particular area. Where the same type device (with equivalent specs.) is available as either 2-wire or 4-wire, the 2-wire device is typically preferred. There are other reasons to choose 4-wire devices depending on the particular
installation. One case may be where there are a cluster of instruments at a particular location somewhat remote from the control system and it may be important to isolate supply power from the devices locally. An alternative in this instance though may also be to use 2-wire transmitters with a local common loop power supply. A lot depends on what is standard in a particular plant and what is
available from approved manufacturers. For the measurement or control signal itself, current signals are typically preferred over voltage signals, when available, due to concerns over voltage drop in long cable runs and the potential to be affected by surrounding noise sources. Depending upon the size and layout of your system, you may also want to consider the use of Hart communications to your devices or one of several
Fieldbus systems now available. Hart and the field busses like Devicenet, Profibus, Foundation Fieldbus etc. typically allow for remote configuration and diagnostics, not available with stand-alone devices.

Robert D. Wagner, P.E.

The above article is good but little more explanation about the advantages and disadvantages will be better to quench the thirst for knowledge.

I have some experience connecting two 2-wire proximity switches in series to drive a relay. They worked when first powered, but after warmed up the voltage drop across the two sensors increase (so we guessed, never measured though). After this happened the relay would no longer latch due to the relays pickup voltage requirement. The prox switch was a 2-wire inductive P&F and I believe the relay was an ice-cube style Omron (or IDEC). Both were 24VDC and did their job correctly, except when wired in this fashion, not recommended.

By poorna chandra raju on 20 February, 2010 - 12:34 am

power supply + connect to transmitter + and transmitter (-) connect to the receiver (+) then receiver (-) to the next receiver (+) up to 15 receivers you have to connect like this finally short the power supply (-) to the receiver (-).i think this will help to you.