Firstly, we need to understand what a transmitter is? A transmitter is an instrument that can convert small signals from sensors into signals that represent the measured variable.

The output signal of the transmitter can be a current signal or a voltage signal.

Due to various reasons, current signal is the most common output signal of transmitters so far. One of the reasons is that if the distance between the transmitter and PLC is too far or the cable is too long, it will cause voltage loss and affect the measurement results. However, current will not attenuate on the line.

The output circuit of a transmitter includes the transmitter, power supply, and receiving device, which can be DCS or PLC. Like other instruments, the transmitter also requires power supply to operate, and its wiring method with the power supply determines whether it is a two-wire or four wire system.

Four wire system

Two wires of the four wire transmitter are connected to the power supply, and the other two signal wires are connected to the PLC. The power supply can be DC or AC, depending on the supplier and model.

Two wire system

As the name suggests, a two-wire transmitter only has two wires.

In a two-wire current circuit, the transmitter, power supply, and PLC are connected in series. These two wires are not only power lines, but also signal lines.

Finally, let's summarize:

1. A transmitter is an instrument that converts small signals emitted by sensors into signals representing the measured variable.

2. The output circuit of a transmitter includes the transmitter, power supply, and receiving device, such as a PLC.

The wiring method between the transmitter and the power supply determines whether it is a two-wire or four wire system.

4. Two wires of the four wire transmitter are connected to the power supply, and two signal wires are connected to the PLC.

5. The two-wire transmitter only has two wires, which are connected in series with the power supply and PLC.