Hi. I’m new to PLCs and I’m having trouble understanding the ladder diagram shown below. The actual problem I’m trying to solve is from a book by W. Bolton called Programmable Logic Controllers 3rd edition and goes as follows:
The inputs from the limit switches and the start switch and the outputs to the solenoids of the valves shown in Figure 10.29 are connected to a PLC which has the ladder program shown in Figure 10.30. What is the sequence of the cylinders?
The correct solution as shown in the back of the book is A+, B+, A-, B-, A+, A-.
I believe I am following the logic correctly for the A+, B+, A-, B- proportion but then I get stuck in a loop where nothing changes (A+ is OFF, B+ is OFF, A- is ON, B- is ON, IR1 is ON, IR2 is OFF, a+ is OPEN, a- is CLOSED, b+ is OPEN and b- is CLOSED). I don’t see how to energize IR2 without first turning off IR1 and in order to turn off IR1, IR2 needs to be turned on.
Like I said I’m new to PLC programming. I imagine the solution is simple but I don’t see it. If anyone could briefly explain what I’m missing I’d really appreciate it. Thanks.
The inputs from the limit switches and the start switch and the outputs to the solenoids of the valves shown in Figure 10.29 are connected to a PLC which has the ladder program shown in Figure 10.30. What is the sequence of the cylinders?
The correct solution as shown in the back of the book is A+, B+, A-, B-, A+, A-.
I believe I am following the logic correctly for the A+, B+, A-, B- proportion but then I get stuck in a loop where nothing changes (A+ is OFF, B+ is OFF, A- is ON, B- is ON, IR1 is ON, IR2 is OFF, a+ is OPEN, a- is CLOSED, b+ is OPEN and b- is CLOSED). I don’t see how to energize IR2 without first turning off IR1 and in order to turn off IR1, IR2 needs to be turned on.
Like I said I’m new to PLC programming. I imagine the solution is simple but I don’t see it. If anyone could briefly explain what I’m missing I’d really appreciate it. Thanks.
