eBook: Automating Manufacturing Systems; with PLCs



A typical machine will use a sequence of repetitive steps that can be clearly identified. Ladder logic can be written that follows this sequence. The steps for this design method are;

1. Understand the process.
2. Write the steps of operation in sequence and give each step a number.
3. For each step assign a bit.
4. Write the ladder logic to turn the bits on/off as the process moves through its states.
5. Write the ladder logic to perform machine functions for each step.
6. If the process is repetitive, have the last step go back to the first.

Consider the example of a flag raising controller in Figure 122 and Figure 123. The problem begins with a written description of the process. This is then turned into a set of numbered steps. Each of the numbered steps is then converted to ladder logic.

Figure 122 A Process Sequence Bit Design Example

Figure 123 A Process Sequence Bit Design Example (continued)

The previous method uses latched bits, but the use of latches is sometimes discouraged. A more common method of implementation, without latches, is shown in Figure 124.

Figure 124 Process Sequence Bits Without Latches

Similar methods are explored in further detail in the book Cascading Logic (Kirckof, 2003).


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