plc ladder programs for logic functions

PLC LADDER PROGRAMS FOR LOGIC FUNCTIONS

The logic functions (such as AND, OR, NOR, etc.) can be obtained by combinations of switches (such as limit switches, solenoid coils, etc.). The following sections show how we can write PLC ladder programs for such combinations.

1. AND Logic Function

Fig.15.21(a) shows a situation where a coil is not energized unless two, normally open, switches are both closed. Switch A and switch B have both to be closed, which thus gives an AND logic situation.

Fig.15.21(b) shows the PLC ladder diagram for the AND logic function shown in Fig.15.21(a). In Fig.15.21(b), the ladder diagram starts with ||, labelled Input 1, to represent switch A and in series with it ||, labelled Input 2, to represent switch B. The line then terminates with O to represent the output.

2. OR Logic Function

Fig.15.22(a) shows a situation where a coil is not energized until either, normally open, switch A or B is closed. This situation is an OR logic gate.

Fig.15.22(6) shows the PLC ladder diagram for the OR logic gate shown in Fig.15.22(a). In Fig.15.22(b), the ladder diagram starts with | |, labelled Input 1, to represent A and in parallel with it | |, labelled Input 2, to represent switch B. The line then terminates with O to represent the output.

3. NOR Logic Function

Fig.15.23(a) shows a NOR logic gate situation. Fig.15.23(b) shows the PLC ladder program for the NOR gate shown in Fig.15.23(a). Since there has to be an output when neither A nor B have an input and when there is an input to A or B the output stops, the ladder program shows Input 1 in parallel with Input 2, with both being represented by normally closed contacts.

4. NAND Logic Function

Fig.15.24(a) shows a NAND logic gate situation. Fig.15.24(b) shows the PLC ladder program for the NAND gate shown in Fig.15.24(a). There is no output when both A and B have an input. Thus for the ladder program line to obtain we require no inputs to Input 1 and to Input 2.