6.5 Bus boards

The board in a row that carries power from the supply to every module. An enclosure row has one or more, behind the panels. Every header on a bus board is an outlet of the bus (6.4): the rails, the header form, the pinout, the marked conductor on −12 V, and the shroud key.

Capacity

A bus board publishes its maximum current on each rail, in total and at each header (PWR-002). The per-header limit it publishes shall not exceed 2 A per rail (BUS-012), the module limit of BUS-008.

EIS · Bus boards: published totals, the per-header limit, regions, and links region A region B CV and gate split here (BUS-013) linked board link module ≤ 400 mm, ≥ 28 AWG (BUS-015) Publishes its maximum current on +12 V, −12 V, and +5 V, in total and per header (PWR-002), never more than 2 A per rail (BUS-012). A link joins CV to CV and gate to gate (BUS-014); every header's 0 V runs to the supply's 0 V (GND-002).

A bus board may split CV and gate into two regions (BUS-013). A link between two bus boards shall join CV to CV and gate to gate (BUS-014). How many modules may transmit on a line is a rule for assembling a system.

The supply-to-bus-board and bus-board-to-bus-board connections are row wiring (6.1): the board's terminals are quick-connect tabs, ring-terminal studs, or a wire-clamp block, published with the conductor sizes they take (PWR-005), and EIS recommends the 6.3 mm quick-connect tab for them.

Distribution on a bus board

A bus board may also distribute a DC rail, the ±6 V analog rails, or the video reference behind the panel, or be the case's CTRL-USB-HOST; it then claims the matching feature, PWR-DC12, PWR-A6, the VID-REF ports, or CTRL-USB-HOST, beside PWR-BUS and meets its requirements (BUS-020). Its PWR-DC12 outlets are those of 6.7, each protected (DC-009, DC-012); its PWR-A6 outlets are A6-VH3 outlets fed from a PWR-A6 supply or a converter on the board (6.8); as a video reference distributor it takes the reference at a VID-REF-IN and gives each module a buffered copy at a VID-REF-PASSTHRU (7.4); as a case host it has the downstream ports and limits of 8.1. The claim line names each feature, bus board (PWR-BUS-OUT, VID-REF-IN, VID-REF-PASSTHRU) or bus board (PWR-BUS-OUT, CTRL-USB-HOST) (4.2). A board that carries no bus at all, only a DC rail or the reference tree, is still a bus board: it claims the features it carries and not PWR-BUS. The board's one 0 V is shared by every domain on it (6.3).

Power ribbon cables

A power ribbon from a bus header to a module shall be no longer than 400 mm and shall use conductors of at least 28 AWG (BUS-015), the conductor of standard 1.27 mm pitch ribbon: the voltage drop of a longer or thinner ribbon at the header limit eats into the module's rails. At 28 AWG (0.21 Ω/m) a 400 mm ribbon drops about 85 mV on a rail carrying 2 A over its two conductors, and a third of that on the 0 V return over its six. A power ribbon cable sold on its own meets the marking and keying of 6.4 (BUS-004, BUS-005). The figures are in data/power_ribbon.json.