Traffic Light¶

A sequential Red → Green → Yellow → Red controller using OTL/OTU latch logic.

Ladder Diagram¶

Rung 0: Initialize — if no state active, latch Red
    ──[/B:0/0 ]──[/B:0/1 ]──[/B:0/2 ]──(L B:0/0 )──
     StateRed   StateYellow  StateGreen    Latch Red

Rungs 1-2: Red → Green transition (manual button)
    ──[ B:0/0 ]──[ I:0/1 ]──(U B:0/0 )──   ; Unlatch Red
    ──[ B:0/0 ]──[ I:0/1 ]──(L B:0/2 )──   ; Latch Green

Rungs 3-4: Green → Yellow transition
Rungs 5-6: Yellow → Red transition

Rung 7: Red light output
    ──[ B:0/0 ]──( O:0/0 )──

Rung 8: Yellow light output
    ──[ B:0/1 ]──( O:0/1 )──

Rung 9: Green light output
    ──[ B:0/2 ]──( O:0/2 )──

I/O Assignment¶

Address	Description
`I:0/1`	Advance button
`B:0/0`	Red state
`B:0/1`	Yellow state
`B:0/2`	Green state
`O:0/0`	Red light
`O:0/1`	Yellow light
`O:0/2`	Green light

Tip

Open examples/traffic-light.plcproj in PiPLC to try this program. You can replace the manual button with timers for automatic sequencing.