Define the Task

A motor is started and stopped from a control station. Pressing Start begins a start sequence: a warning horn sounds for two seconds to clear the area, then the motor contactor pulls in and a green run light comes on. Pressing Stop drops everything immediately, at any point in the sequence. If the motor overload trips, everything drops as well and cannot be restarted until the overload is reset. The motor must never restart on its own — not after a stop, not after an overload reset, and not when control power comes back.

Define the Inputs and Outputs

X1 = Start pushbutton, wired N.O. — OFF at rest, ON while pressed. X2 = Stop pushbutton, wired N.C. — ON at rest, OFF when pressed. X3 = motor overload auxiliary contact, wired N.C. — ON while the motor is healthy, OFF when tripped. Y1 = motor contactor. Y2 = green run light. Y3 = warning horn.

Both field contacts are wired normally closed on purpose. A cut wire or a loose terminal on either one reads the same as a pressed Stop or a tripped overload, so the failure stops the motor instead of stranding it running. Read the state, not the label: X2 and X3 are ON when the machine is in its normal condition, so in the program they are examined with normally-open contacts.

Develop the Sequence of Operation

Pressing Start with X2 and X3 both ON sets a start request that must seal itself in, because X1 is released long before the motor runs. The start request drives an on-delay timer with a 2-second preset. While the request is on and the timer has not yet timed out, the horn Y3 is on and the contactor Y1 is off. When the timer times out, the horn drops and Y1 pulls in; Y2 follows Y1. Losing X2 or X3 clears the start request, which also resets the timer — so a Stop pressed one second into the warning aborts the start cleanly rather than queueing it.

Work out what happens when Start is pressed and held. Then work out what happens if Start is pressed again while the motor is already running.

Develop the PLC Program

Build it in the ACC PLC Simulator. A workable structure is five rungs: (1) a seal-in rung — X1 in parallel with the seal contact, in series with X2 and X3, driving an internal bit for the start request; (2) the start request driving an on-delay timer with a 2-second preset; (3) the start request AND the timer not done driving Y3; (4) the timer done driving Y1; (5) Y1 driving Y2. The one point to get right is that the timer is enabled by the sealed-in start request, not by X1 directly — a momentary press is far shorter than the delay.

Comment every rung. The seal-in rung in particular should say what it is sealing and what breaks it.

Test the Program

Connect this scene, press RUN, and verify: (1) Start sounds the horn for two seconds, then the contactor pulls in and the run light comes on; (2) Stop pressed during the warning aborts the start with no motor run; (3) Stop pressed while running drops Y1 and Y2 immediately; (4) tripping the overload drops everything and blocks a restart until reset; (5) resetting the overload does not restart the motor on its own; (6) holding Start does not skip or shorten the warning.

Then verify the addresses from outside the browser. Point a Modbus master at the bridge and confirm DI 10001–10003 track the pushbuttons and the overload, and that coils 00001–00003 follow the contactor, run light and horn. Use STEP mode to trace the timer rung scan by scan.