In part 1 we looked at writing PLC programs to control a traffic light using discrete bits and then using timed sequencing using indirect addressing. We will now look at how we can use indirect addressing for inputs as well as output to control the sequence in the program.
Lets look at an example of controlling pneumatic (air) cylinders.
This site contains a video of the three cylinders and the sequence required.
This program will have the following inputs. Even thought no sensors are mounted on the cylinders, it is best to have sensor inputs when the cylinder is extended (out) and retracted (in)
Cylinder 1 In – X1
Cylinder 1 Out – X2
Cylinder 2 In – X3
Cylinder 2 Out – X4
Cylinder 3 In – X5
Cylinger 3 Out – X6
Start PB NO – X7
Stop PB NO – X8
Step PB NO – X9
This program will have the following outputs.
Cylinder 1 In – Y1
Cylinder 1 Out – Y2
Cylinder 2 In – Y3
Cylinder 2 Out – Y4
Cylinder 3 In – Y5
Cylinger 3 Out – Y6
We will use the following pointers:
V0 – Output pointer starting at address V2000
V1 – Input pointer starting at address V1000
V10 will be the input word
V20 will be the output word
Before we start and write the code lets look at the sequence that we are trying to accomplish. The best way to do this is a chart indicating the inputs and output. I use either graph paper or a spreadsheet software to configure the sequence.
I usually start with the outputs configure the sequence that I would like to see. Then based upon the output sequence, I figure out the input sequence.
Once the sequence has been established, the next step is writing the program.
Input program that will set the input bits in V10.
Control part of the program:
The first scan will reset the input and output pointers.
The input pointer is compared to the input word V10. If they are equal then the output pointer and input pointer are incremented. If the STEP input is hit, then the output and input pointers are incremented.
The output pointer is then compared to the maximum value (end of sequence). If it is greater than or equal to the maximum value then the pointers will be reset.
Line 12 will move the outputs indirectly to the output word.
Output program that will set the actual outputs based upon the bits in V20
As you can see the actual program is very small however the sequence can be thousands of steps. This is a very straight forward and powerful method of programming. Programming this sequence using bits, timers and no indirect addressing would be very difficult and hard to read. Modifications would have to be a complete re-write of the program.
The entire program sequence could change without further lines of code. Only the values in the registers would need to be modified. This could lead to different sequences for different products.
We used a step input to have the program move forward through the sequence. It would be just as easy to add a step reverse function for the program. We would just have decrement the pointers and check to make sure when we were at the beginning of the sequence.
When troubleshooting this program we would only need to look at the compares to determine what input and or output is not working correctly.
Integration with a touch panel display is simplified when using this type of programming method.
What other advantages do you see?
In Part 3 we will build on the traffic light sequencing used in part one with inputs for pedestrian and car detection.
Contact me for the above program. I will be happy to email it to you.
If you have any questions or need further information please contact me.
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If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.
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