Signals are physical quantities that identify messages. For example, electrical signals can represent different messages through changes in amplitude, frequency, and phase. There are two types of electrical signals: analog signals and digital signals. Signals are tools for carrying messages and are the carriers of messages.
In the process of using instruments, current signals, voltage signals and digital signal lights are used. For current signals, if the instrument has an independent working power supply, the signal output it provides is an active signal. If the instrument does not have an independent power supply, the signal it provides is a passive signal. Instruments that use three-wire voltage signals and four-wire digital signals are active signals.
Active signal acquisition:
The acquisition of active signals from instruments is relatively convenient. First, turn on the power of the signal source device (generally there are two types: three-wire and four-wire) to allow the equipment to work normally. Second, connect the positive and negative ends of the input interface of the acquisition device to the positive and negative ends of the signal source device respectively.
Passive signal acquisition:
To collect active and passive signals of instruments and meters, the first step is to use a DC 24V power supply (because the two-wire instruments and meters that provide passive signals do not have a separate working power supply, when it needs to output a 4-20mA signal, its working voltage needs to be provided by an external device.
This DC24V power supply can be an independent power supply, or it can be provided by the power distribution function of the passive signal acquisition equipment); the second step is to generate a correct circuit between the two-wire instruments and meters, the DC24V power supply and the passive signal acquisition equipment: the positive pole of the DC24V power supply is connected to the positive pole of the 4-20mA output of the two-wire instruments and meters, the negative pole of the 4-20mA output of the two-wire instruments and meters is connected to the positive pole of the 4-20mA signal input terminal of the acquisition equipment, and the negative pole of the 4-20mA signal input terminal of the acquisition equipment is connected to the negative pole of the DC24V power supply, thus generating a complete circuit.