Marshalling Cabinet.

A marshalling cabinet is the deliberate seam between two things that change on different schedules. The field cabling, which arrives in whatever order the cable schedule dictates, and the controller I/O cards, which have a fixed channel layout. Field cables land on terminal blocks on one side of the cabinet, short cross-wires carry each one across to the system-side terminal that leads to its I/O channel, and that cross-wired layer is where the project's specific tag-to-channel assignment is imposed. Without it the field wiring would have to match the card layout perfectly from the first day, and any later change, swapping a card type, moving a tag to a different signal class, adding a spare would mean re-pulling cable in the field instead of re-landing a cross-wire in a cabinet. The cabinet is sized from the I/O list. The channel count by signal class, plus the spare allocation a project carries, commonly around twenty percent, plus any segregation rule that keeps SIS apart from BPCS, together set the terminal count, the rack space, and ultimately the number of cabinets. Smart-marshalling systems change the economics by making each channel software-configurable as any signal class rather than fixed by card, which collapses the cabinet count and absorbs late I/O changes without rewiring, at a higher cost per channel that large projects justify on schedule and change-management grounds. Either way the starting point is a complete, signal-classified count of what the drawings carry.