Hysteresis: the analog seal-in

Wire the obvious rung — AI1 ≥ 60 → pump — and you've built a machine for destroying contactors. Here's why, and the two-contact fix that every professional analog circuit carries.

One setpoint = chatter

A real signal is never still. Ripples in the tank, electrical noise, the resolution of the card itself — the reading breathes a percent or two even when the process is steady. Park that breathing right on your single setpoint and the compare flips with every wiggle:

The top chart is the obvious rung: the level grazes 60 and the pump snaps ON-OFF-ON-OFF — chatter. Each snap is a contactor slam, a current inrush, a water hammer in the pipe. Motors and valves die of this, not of running.

The fix is a seal-in — around a compare

You already know the cure from the discrete world: the output holds itself in through its own contact. The analog version is the same move with a second, LOWER compare as the stop:

      ┌──[ AI1 ≥ 60 ]──┐
──────┤                ├──────[ AI1 > 30 ]──────( M1 pump )
      └─────[ M1 ]─────┘

Walk it:

• Below 30 — both compares false, pump off. Rest state.
• Rising through 60 — the top branch conducts, M1 energizes, and the seal contact closes around the start compare.
• Falling back to 45 — AI1 ≥ 60 is false again, but the rung stays alive through M1 · AI1 > 30. The pump keeps pulling the tank down.
• Reaching 30 — the series compare opens, the seal dies, pump off. The tank must climb all the way back to 60 to restart it.

Between 30 and 60 the rung's answer depends on which direction you came from — the rung remembers. That memory is called hysteresis, and the 30-point gap is the deadband.

How wide is a good deadband?

Wide enough that noise can't cross it: if the signal breathes ±2%, a 3-point band still chatters. Narrow enough that the process stays in spec: a tank allowed to swing 30 to 60 is fine for sump duty, terrible for a supply header. The width is a process decision, not a logic one — the logic is always this same rung.

Feel the memory

Open the tank circuit. Slide AI1 to 65 — P1 starts. Now slide down to 45 and hold: still running. Keep going to 25: it lets go. Climb back to 45: still off. Same number, two different answers — that's hysteresis, and now you can read it straight off the print.