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A more common method of implementation, without latches, is shown in Figure

10.4.

plc design -10.6

step1

step2

step3

step4

down motor

up motor

Figure 10.4 Process Sequence Bits Without Latches

10.3 TIMING DIAGRAMS

Timing diagrams can be valuable when designing ladder logic for processes that are only dependant on time. The timing diagram is drawn with clear start and stop times. Ladder logic is constructed with timers that are used to turn outputs on and off at appropriate times. The basic method is;

1. Understand the process.

plc design -10.7

1. Identify the outputs that are time dependant.
2. Draw a timing diagram for the outputs.
3. Assign a timer for each time when an output turns on or off.
4. Write the ladder logic to examine the timer values and turn outputs on or off.

Consider the handicap door opener design in Figure 10.5 that begins with a verbal description. The verbal description is converted to a timing diagram, with t=0 being when the door open button is pushed. On the timing diagram the critical times are 2s, 10s, 14s. The ladder logic is constructed in a careful order. The first item is the latch to seal-in the open button, but shut off after the last door closes. auto is used to turn on the three timers for the critical times. The logic for opening the doors is then written to use the timers.

plc design -10.8

Description: A handicap door opener has a button that will open two doors.

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