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PLC Registers

We'll now take the previous example and change switch 2 (SW2) to a normally closed symbol (loadbar instruction). SW1 will be physically OFF and SW2 will be physically ON initially. The ladder diagram now looks like this:

Completed ladder diagram

Notice also that we now gave each symbol (or instruction) an address. This address sets aside a certain storage area in the PLCs data files so that the status of the instruction (i.e. true/false) can be stored. Many PLCs use 16 slot or bit storage locations. In the example above we are using two different storage locations or registers.

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
1 0
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

In the tables above we can see that in register 00, bit 00 (i.e. input 0000) was a logic 0 and bit 01 (i.e. input 0001) was a logic 1. Register 05 shows that bit 00 (i.e. output 0500) was a logic 0. The logic 0 or 1 indicates whether an instruction is False or True. *Although most of the items in the register tables above are empty, they should each contain a 0. They were left blank to emphasize the locations we were concerned with.

Logic 0 False True False
Logic 1 True False True

The plc will only energize an output when all conditions on the rung are TRUE. So, looking at the table above, we see that in the previous example SW1 has to be logic 1 and SW2 must be logic 0. Then and ONLY then will the coil be true (i.e. energized). If any of the instructions on the rung before the output (coil) are false then the output (coil) will be false (not energized).

Let's now look at a truth table of our previous program to further illustrate this important point. Our truth table will show ALL possible combinations of the status of the two inputs.

Inputs Outputs Register Logic Bits
False True False 0 0 0
False False False 0 1 0
True True True 1 0 1
True False False 1 1 0

Notice from the chart that as the inputs change their states over time, so will the output. The output is only true (energized) when all preceding instructions on the rung are true.

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