When the fault has no wire: follow the cross-reference

When the false condition is an internal bit, not a physical input, the cross-reference tells you exactly which rung writes it. Follow the map and the trail always ends at a real device.

A faultline101 training module · the skill behind six hands-on scenarios

When the false condition is an internal bit instead of a physical input, the cross-reference names the exact rung that writes it — and the trail always ends at a real device.

The idea

Not every condition on a rung is a physical input. Plenty are internal bits — permissives, ready flags, status bits — set by other rungs. When one of those is false, you can't walk out to the field yet; you have to find where it's written first.

That's what a cross-reference is for: it lists every place a tag is used, and crucially tells a coil (where the bit is written) from a contact (where it's read). Jump to the rung that writes the false bit, and you're one step closer to a real device.

Two traps the cross-reference kills: a bit written in two rungs (a destructive double-write that makes it flicker), and a bit written in a completely different routine. Without the cross-reference you'd stare at the wrong rung for an hour; with it, you follow the map straight to the source.

Six faults you'll work in this module

You don't learn this by reading it — you learn it by doing it on six different faults, each drilling the same skill until it's instinct:

1. The false condition with no wire. Internal bit to physical fault, in two clicks.
2. Who actually writes this bit?. The map named the exact rung — now read its conditions.
3. Written in two places. Two coils, one bit: a scan-order flicker the map exposed.
4. Coil or contact?. The coil is the cause; the contact is just the effect.
5. The source hid in a subroutine. The source hid in another routine until the map found it.
6. Cold trace through three bits. No machine knowledge needed — the map did the work.

The mistakes it kills

• Staring at the rung where a bit is READ instead of jumping to where it's WRITTEN
• Missing that a bit is written in two places, so you fix one and the other undoes it
• Not realizing the source rung lives in a different routine or subroutine

Why it matters: The cross-reference is what keeps you from getting lost the moment a fault leaves the physical I/O and hides in internal logic. It turns 'where does this bit come from?' into a two-click answer.

More: Follow the rung backward · Get the comic · All troubleshooting guides.