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1. Input-side loop-powered type: For this isolation technique, the electrical power required for the input and output circuits comes from the input side; power is supplied via a loop circuit. The loop current is typically a standard signal of 4–20 mA, which serves as the input signal (for example, from a PLC output card or the output of a four-wire transmitter). After being isolated, this input signal is sent to actuators, regulators, or other devices through the output side. The loop power isolation device requires no additional auxiliary power supply, making it easy to use. For example, Stabil-Tek’s TA2000 is a passive signal isolator of the input-loop powered type. 2. Isolated power supply for the output circuit: With this isolation technique, the electrical energy required for the input and output components comes from the output side; the output side obtains its power through a loop-powered system (for example, a PLC input card). The loop current is typically a standard signal of 4–20 mA, which is used as the output signal. The input can be signals from various sensors, or it can be a 4–20mA standard signal. An important feature of this technology is that the signal current flowing to the output not only powers the isolation unit but also supplies the load of the isolator. (For example, the input resistance of a PLC input card). Typical models such as the Stabiltek TA2001 passive isolator provide isolation for the power supply of the output circuit. For this type of isolation technology, the electrical energy required by the input and output components comes from the output side; the output side obtains its power through circuit supply (for example, PLC input cards). The circuit current is usually a standard signal of 4–20 mA, which is used as the output signal. As an example, the Stabil-Tek TA2002 passive power distribution isolator (which belongs to passive signal isolators)