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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 10.2 and Figure 10.3. 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.

plc design -10.4

Description: A flag raiser that will go up when an up button is pushed, and down when a down button is pushed, both push buttons are momentary. There are limit switches at the top and bottom to stop the flag pole. When turned on at first the flag should be lowered until it is at the bottom of the pole.

Steps:

1. The flag is moving down the pole waiting for the bottom limit switch.
2. The flag is idle at the bottom of the pole waiting for the up button.
3. The flag moves up, waiting for the top limit switch.
4. The flag is idle at the top of the pole waiting for the down button.

Ladder Logic:

first scan

step 1 step 2 step 3 step 4

step 1 down motor

step 1 bottom limit switch step 2

The ladder logic for step 1 turns on the motor to

step 1 lower the flag and when the bottom limit switch is hit it goes to step 2.

step 2

Figure 10.2 A Process Sequence Bit Design Example

plc design -10.5

hit it goes to step 4. step 4 flag down button step 1 The ladder logic for step 4 only waits for the

step 4 push button to lower the flag.