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: interlock + limits + jog

The print takes the bare pattern and makes it a machine — a carriage that runs forward to a limit, returns home, and can be jogged by hand:

• 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.

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. Then try to force both motors on at once — the interlock won't let you. Flip to manual and jog past a limit to see what the bypass really bypasses.