Ladder Logic, Interlocks & State Sequences

The seal-in rung

The most important rung in industrial control starts and stops a motor from two momentary pushbuttons ¹:

 Start      Stop       Motor
--| |---+---| |--------( )--
        |
 Motor  |
--| |---+

• Press Start: its contact closes, the Motor output turns on.
• The Motor contact in parallel with Start (the seal-in, or latch) closes too, so when you release Start, the rung stays powered through it.
• Press Stop: the rung breaks, the output drops, the seal-in opens, and the motor stays off until Start is pressed again.

Notice the Stop contact is drawn normally open in the program, which looks backwards. That’s because the real stop button is wired normally closed: when nobody is pressing it, the input is on, the program contact is closed, and the motor can run. Pressing it, or a broken wire, turns the input off and stops the motor. Wire the stop button the other way and a broken wire would leave the machine with no way to stop it from that button ¹.

A side benefit of the seal-in: after a power outage, the motor doesn’t restart by itself when power comes back. Someone has to press Start.

Permissives and interlocks

A permissive is a condition that must be true before an action may start. An interlock prevents or stops an action when something incompatible or dangerous is true.

For the infeed conveyor, the run command might need all of these in series:

Auto_Mode · Drill_Station_Ready · Drive_Ready · Guards_Closed · No_Jam · No_Fault · Run_Request

Don’t collapse these into one bit called Ready. When the conveyor won’t start, the operator’s question is why, and named intermediate tags (Drive_NotReady, Jam_Detected) answer it on the HMI without a laptop.

A classic interlock: a reversing motor has a forward contactor and a reverse contactor, and both on at once is a dead short. Each rung includes a normally closed contact from the other output, so forward can’t energize while reverse is on, and vice versa ¹. Reversing starters commonly add a mechanical interlock between the contactors, because the program shouldn’t be the only thing standing between you and a short.

A sequence with timeouts

The drill station does the same steps every cycle. Write them as states, one active at a time, each with an action, a condition to move on, and a timeout:

Every waiting step ends one of two ways: it completes, or it times out to a fault state that names the step. Without timeouts, a stuck sensor leaves the machine sitting silently in step 2 and someone spends twenty minutes guessing. IEC 61131-3’s sequential function chart language is built for exactly this step-and-transition structure ².

What a fault state does depends on the hazard: hold the clamp closed so the part doesn’t drop, retract the drill, stop the conveyor. Decide it per step, and decide what the operator must do to recover.

Alarms that help

A good alarm text says what happened and where to look: “Clamp did not close in 2 s: check air pressure and clamp-closed switch LS-201.” Show the first-out alarm (the root cause) above the pile of follow-on alarms it triggered. Reset should clear the alarm once the condition is fixed, and should never restart motion by itself.

Safety stays separate

The PLC program can ask for a stop, but an ordinary PLC isn’t the safety system. Emergency stops, guard interlocks, and light curtains run through safety-rated relays or safety PLCs designed and validated for that job, as OSHA’s guidance on robot systems describes ³. The standard PLC watches the safety circuit’s status so it can show what tripped and sequence a clean restart.

Try it

Write the rungs for the infeed conveyor: a seal-in with an NC stop, the seven permissives above, and a fault that latches if RunCmd is on for 3 seconds without RunningFb. Then add a step 2½ to the drill sequence that checks air pressure before drilling, with its own timeout and alarm text.