Forward / reverse without smoke

A motor that runs a carriage forward and backward has two contactors — and if both close at once, you've wired a short across the incoming phases. The result is smoke, welded contacts, and a very bad day. The interlock pattern makes "both at once" impossible in the logic, before it can happen in the panel.

The core: each direction locks out the other

   FwdRequest    Rev
──────[ ]───────[/]──────────( Fwd )
   RevRequest    Fwd
──────[ ]───────[/]──────────( Rev )

Each rung examines the other output with an XIO. Whoever energizes first opens the other rung — the loser can't win until the winner drops out. Ask for both at once and the scan order breaks the tie: rung order is the referee.

Real panels back this up with a mechanical interlock on the contactors themselves. The logic interlock is not a substitute — it's the first of two layers, and it's the one you're responsible for in the program.

The forward/reverse print: limits + jog

Here is a real print from the lab — a carriage that runs forward to a limit, returns home, and can be jogged by hand. Read what it actually contains, in order:

• A sealed START_BIT (the internal relay from the last lesson) arms the automatic mode.
• Forward runs while the carriage is at home (IP1) and stops at the forward limit (/IP3 in series).
• Return starts from the forward limit and stops at home (/IP1) — the same limit that allowed forward now ends reverse.
• Jog is a two-contact branch — manual mode AND the jog switch — that bypasses the automation entirely, exactly as printed.

The limits do double duty: each one is a "you're here" sensor for one direction and a "stop now" for the other. That's the economy of a good print — every input earns its terminal block.

Read the print before you trust it

Now go back and look for the interlock in that list. It isn't there.

This print has no logic interlock. Neither motor rung examines the other output — no [/] on Rev in the Fwd rung, none on Fwd in the Rev rung. In automatic mode the limits keep the two directions apart: forward stops at IP3, and reverse can only start from IP3. That works, but it is a different mechanism with a different failure surface. In manual mode there is nothing at all: jog and return-home are two separate branches hanging off the manual selector, and neither one knows the other exists.

That is not a mistake in the print. On a real reversing starter the interlock lives in two places you can touch: the contactor's mechanical latch, and its auxiliary contacts wired into the opposite coil. A designer who has those may decide the program doesn't need a third copy. Whether you agree is the argument worth having — but you can only have it after you've read what's actually on the page.

Compare the star-delta starter in the same builder: there the interlock is in the logic ([/] on each contactor in the other's rung), because nothing upstream provides one. Same catalog, opposite decision, and both are defensible for their own panel.

Where people get bitten

The jog branch bypasses more than you think. In the print, jog rejoins the rung after the forward limit — so jog can push the carriage past the limit on purpose (that's what jog is for: recovery and setup). Read carefully what a bypass branch skips; the print is telling you what the designer decided jog is allowed to do.

Run the real print

Load the forward/reverse circuit in the builder. Arm auto with SS1, set the home limit IP1, and start — watch the limits hand the carriage back and forth without either rung ever mentioning the other motor.

Then flip to manual and prove the gap for yourself: hold jog and set-home at the same time. Both motor outputs energize. On the bench that is two contactors closing across the incoming phases — the exact thing the pattern at the top of this lesson exists to prevent. The simulator will let you do it, because the print lets you do it.

Now add the two contacts yourself: an XIO on M2 in the forward rung and an XIO on M1 in the reverse rung. Try the same thing again. That difference — three seconds of editing — is the whole lesson.