In a low-pressure condition, the energized input I:0/2 forces the virtual normally-closed contact I:0/2 to open, thus de-energizing the PLC’s output O:0/1 and turning the alarm lamp off. The effect is the same: the PLC output O:0/1 will activate whenever input I:0/2 de-energizes (whenever the pressure switch is opened by a high pressure), turning on the alarm lamp in a high-pressure condition. With virtual coil O:0/1 “unpowered,” the real-life output O:0/1 on the PLC will be electrically open, and the alarm lamp will be unpowered (off). This means the normally-closed contact B3:0/0 will now be “actuated” and thus in the open state, not sending virtual power to the output coil O:0/1. “Energizing” this virtual coil has the effect of “actuating” any contact drawn in the program bearing the same label. This “virtual” contact sends virtual power to a virtual coil labeled B3:0/0, which is nothing more than a single bit of data in the PLC’s microprocessor memory. Thus, the closed pressure switch contact energizes input terminal I:0/2, which in turn “closes” the normally-open contact symbol I:0/2 drawn in the ladder-logic program.
The contact labeled I:0/2 drawn in the ladder-logic program of the PLC acts like a relay contact driven by a coil energized by input terminal I:0/2. This sends power to input I:0/2 of the PLC. This is less than the switch’s trip setting of 50 PSI, leaving the switch in its “normal” (closed) state.
Suppose a fluid pressure of 36 PSI is applied to the pressure switch.