Introduction to PLC Programming and Implementation (7)

EXAMPLE 1
For the circuit shown in Figure 7, (a) identify the real inputs and outputs by circling each, (b) assign the I/O addresses, (c) assign the internal addresses (if required), and (d) draw the I/O connection diagram.

Assume that the PLC used has a modularity of 8 points per module. Each rack has 8 module slots, and the master rack is number 0. Inputs and outputs can have any address as long as the correct module is used. The PLC determines whether an input or output module is connected in a slot. The number system is octal, and internals start at address 10008.


SOLUTION
(a) Figure 8 shows the circled real input and output connections. Note that temperature switch TS3 is circled twice even though it is only one device. In the address assignment, only one of them is referenced, and only one of them is wired to an input module.
(b) Table 4 illustrates the assignment of inputs and outputs. It assigns all inputs and all outputs, leaving spare I/O locations for future use.

(c) Table 5 presents the output assignments, including a description of each internal. Note that control relay CR2 is not assigned as an internal since it is the same as the output rung corresponding to PL1. When the control program is implemented, every contact associated with CR2 will be replaced by contacts with address 020 (the address of PL1).

(d) Figure 9 illustrates the I/O connection diagram for the circuit in Figure 7. This diagram is based on the I/O assignment from part (b). Note that only one of the temperature switches, the normally open TS3 switch, is a connected input. The logic programming of each switch should be based on a normally open condition.

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