One relay board, found and claimed afresh for the duration of every call.

Built by Arb.board/2 or Arb.list_boards/1. A board holds a selector — not a device and not a USB claim — so it is free to build, resolves nothing until an operation is called on it, and may outlive or predate the device it names. Several boards, in this node or in another application, can drive the same hardware.

Atomicity

One operation is atomic — a single claim spans all of it, read-back included — but a sequence of them is not. Arb.relays/1 followed by Arb.set_relays/3 is two claims, so on a board shared with another application a write can land in the gap and be silently lost:

# Racy on a shared board: a write between these two calls is overwritten.
{:ok, active} = Arb.relays(board)
:ok = Arb.set_relays(board, [3 | active])

The board latches all eight relays at once, so there is no partial update to reach for instead. Where the hardware is shared and read-modify-write is unavoidable, serialise it outside this library.