Section 10 · Practice Questions

Traffic Light Control

One problem on PLC-based traffic signal control — a fixed RED → GREEN → YELLOW sequence with a one-time power-up safety phase using retentive memory, so the initial 15-second RED runs exactly once over the entire lifetime of the code.

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A traffic-light controller is two programs in one — a one-shot startup phase guarded by a retentive bit, and a cyclic state machine that loops forever after the startup completes. Separating the two keeps the logic readable.

Q1

Traffic light cycle with a once-in-a-lifetime startup RED

Write a PLC program that controls the traffic lights for one direction according to a fixed sequence (RED → GREEN → YELLOW → RED). Provide an initial safety feature: when the signal system is powered up, it must always begin from RED — but only once over the entire lifetime of the code (the special initial RED runs only on the very first power-up). On that first power-up the initial RED phase must last 15 seconds and must run exactly once; thereafter the normal cycle repeats indefinitely.

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Strategy

The phrase “once over the entire lifetime of the code” is the key — a normal first-scan bit (S:1/15) fires every time the PLC is powered up, which would re-run the 15-second RED on every restart. The fix is a retentive bit FIRST_RUN_DONE stored in non-volatile memory: on power-up the rung checks “is FIRST_RUN_DONE still zero?” — if yes, run the startup RED and then latch the bit forever; if no, skip straight to the normal cycle. The normal cycle is a three-state machine (RED 30 s → GREEN 25 s → YELLOW 5 s).

Ladder program 
Rung 1 — special first-run 15-s RED, only when FIRST_RUN_DONE = 0:
|---[ S:1/15 ]---[/FIRST_RUN_DONE]----+----( STARTUP_RED )----|
|                                     |
|---[ STARTUP_RED ]---[/T_INIT.DN]----+

Rung 2 — 15-second initial timer:
|---[ STARTUP_RED ]----(TON T_INIT, PRE = 15 s)----|

Rung 3 — mark first-run as done, retentive latch:
|---[ T_INIT.DN ]----(L FIRST_RUN_DONE)----|

Rung 4 — normal sequence runs after STARTUP_RED drops:
|---[ FIRST_RUN_DONE ]----( NORMAL )----|

Rungs 5–7 — cyclic state machine (30 s RED → 25 s GREEN → 5 s YELLOW):
|---[ NORMAL ]---[/T_R.DN]----( RED_PHASE )-----|
|---[ NORMAL ]---[ T_R.DN ]---[/T_G.DN]----( GREEN_PHASE )---|
|---[ NORMAL ]---[ T_G.DN ]---[/T_Y.DN]----( YELLOW_PHASE )--|

Rung 8 — phase timers:
|---[ RED_PHASE    ]----(TON T_R, PRE = 30 s)----|
|---[ GREEN_PHASE  ]----(TON T_G, PRE = 25 s)----|
|---[ YELLOW_PHASE ]----(TON T_Y, PRE =  5 s)----|

Rung 9 — cycle restart at the end of YELLOW:
|---[ T_Y.DN ]----(RES T_R)----|
|---[ T_Y.DN ]----(RES T_G)----|
|---[ T_Y.DN ]----(RES T_Y)----|

Rung 10 — physical lights:
|---[ STARTUP_RED ]----+----( RED )-----|
|---[ RED_PHASE   ]----+
|---[ GREEN_PHASE  ]------( GREEN )-----|
|---[ YELLOW_PHASE ]------( YELLOW )----|

FIRST_RUN_DONE is a retentive bit — it survives power cycles, so the startup RED never repeats.

Timing diagram (after first power-up)

RED : |‾‾‾ 15 s STARTUP ‾‾‾| |‾‾‾ 30 s ‾‾‾| |‾‾‾ 30 s ‾‾‾| GREEN : |‾‾‾ 25 s ‾‾‾| |‾‾‾ 25 s ‾‾‾| YELLOW: |5 s| |5 s| t →

On any subsequent power-up FIRST_RUN_DONE is already 1, so the startup row is bypassed entirely and the lights pick up at the start of the normal RED phase.

Final AnswerA retentive FIRST_RUN_DONE bit guards the 15-second startup RED; once it latches, the cyclic 30 s RED / 25 s GREEN / 5 s YELLOW state machine runs forever. The startup RED never repeats unless the retentive memory is cleared.