This relay replacement example uses the PLC implementation for the electromechanical circuit in Figure 23.
Figure 23: Electromechanical relay circuit.
Hardware timer TMR1 requires instantaneous contacts in the first rung, which are used for latching the rung. If instantaneous TMR1 contacts were implemented using a PLC timedelay contact, then PB1 should be pushed until the timer has set to latch the rung. This instantaneous contact is implemented by trapping a timer with an external output.
Tables 8 and 9 give the I/O address as well as the internal output assignments for an electromechanical circuit’s true I/O. The register assignment table is shown in Table 10. The internals cannot replace the control relays CR1 or CR2, since output addresses 030 and 031 that correspond to solenoids (SOL1 and SOL2) are available.
Addresses 030 or 031 can be used to replace the CR1 or CR2 contacts wherever they appear in the program. The normally open contact LS1 connects to the PLC input interface. The normally open LS1 refers to the normal closed LS1 program. The PLC program coding solution is illustrated in Figure 24.
Figure 24: PLC implementation of the circuit in Figure 23.
