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Note that in order to run this program you must call up the input, mask, and output tables and write them to the simulator or PLC. This program and the data tables can be downloaded here. The program will not change much for completely different sequences.
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The ‘do not walk signal’ is not part of the sequence but is controlled when the flashing walk or walk is not on. This way will give me the greatest flexibility when developing different sequences. You will note that the flashing green lights are done when both green outputs are not on. The actual outputs are set using the output word bits. Outputs – Building a PLC Program – Traffic Light If they are equal then move the output table word to the output channel and increment the pointers to the next step. We now compare the actual inputs after the mask with the input table word. On this step, we then determine if the pointers need to be changed for the other two. You will notice that the first three sequences are all the same. This is used to ignore the inputs that we do not want to see or may not know the status during the execution of the program. The table input pointer is compared to the last value +1 of the sequence running. You will see that since we have three different sequences running, there are three different reset rungs in parallel. It resets the pointers and moves the initial output setting to the output word. This section of the control will tell the PLC what to do when the unit is first powered on. This sets the minimum time between each step.Ĭontrol – Building a PLC Program – Traffic Light The program is all controlled by one on-delay timer. Inputs – Building a PLC Program – Traffic Light
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The pedestrian walk signals flash before they change to walk signals. All bits without ‘1’ are assumed to be ‘0’. This makes it easier to read how the lights will behave. Note that I have color-coded the outputs that will be on in the sequence. Let’s look at the sequence that we are controlling – Building a PLC Program – Traffic Light Part 4 will now continue with the programming of the logic in the PLC. We looked at the sequence of operations using Input, output, and mask tables. Part 3 returned to the traffic light application and expand our program significantly. Part 2 used indirect addressing for inputs as well as output to control the sequence of pneumatic (air) cylinders in the program. 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.