Half-duplex, one packet at a time. A signal enters from my side (−1), crosses or passes at transcriber 1, is held at the seam (0), crosses or passes at transcriber 2, and exits toward your side (+1). A switch in cross is the 180° swap (NEG); in pass it's the 0° hold. The law the test pins down: even crosses send it home, odd crosses leave it on the far side — two crosses close the turn. Set the two switches, fire a packet, watch it cross and land. The spec it was frozen against runs live on the right.
LiteralThe device is exact: cross=NEG, pass=identity, seam=0; parity law (even crosses home, odd away); two crosses = full turn = identity; one packet in the seam at a time. All 10 checks re-run in your browser.
BridgeReading the two switches as the two transcribers / two half-turns, and the held seam as the shared pivot between them.
SpeculativeThis is v1 — half-duplex, one stream. The simultaneous (full-duplex) version where both sides cross the seam at once is a different, harder machine needing the 90° to avoid collision. Not built here.