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plc wiring -2.5

push buttons

power supply +24V

com.
PLC
inputs
ladder
logic A B C
outputs
115Vac light
AC power
neut.
Figure 2.3 A PLC Illustrated With Relays

Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure 2.4 is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off.

plc wiring -2.6

Note: The line on the right is being left off intentionally and is implied in these diagrams.

Figure 2.4 A Seal-in Circuit

2.1.2 Programming

The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer -but, this method has stuck and it is the most common technique for programming PLCs today.

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