Answer / ranjith_asp

As the name suggests, these instruments will have 4 wires
connected to its Terminal Block. Take a Pressure Transmitter
for example
- 2 wires are for Power Supply. Typically 24VDC. This is to
power the instrument to make it work. These 2 wires will be
routd via a JB and then connected to a power supply module
in the PLC cabinet.
- remaining 2 wires are for 4-20mA signal transmission. This
output signal will be proportional to the measured Pressure.
These 2 signal wires will be connected to the AI card in the
PLC cabinet.

Answer / hadi

4 wire connection sensor have two methods:
1-sensor's power is provided from card
2-sensor's power is provided from field
in first method the power is +24VDC and the current limit
exist but in second method the power is every thing and
transmitter feed from external power supply.
in two methods the signal lines and power lines are different.

Answer / ramaswamy and karuppusami

Usually in 2wire transmission, 24v supply is given and o/p
in mA are taken through single pair cable. e.g: Transmitters.

But in 4wire system, supply is provided in separate single
pair cable and o/p is taken through another cable. e.g :
local speed indicator device: i/p = 230v and o/p = 4-20
mA(Both are in separate cables ).

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