6.5 Bus boards
The board in a row that carries the bus from the supply to the modules that take it at a power header. A row with the bus has one or more behind the panels, and an enclosure may build them in. Every header on a bus board is an outlet of the bus and meets 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 a bus board, a built-in bus board of an enclosure, or a power supply publishes for its power-header outlets shall not exceed 1.5 A per rail (BUS-012), the module limit of BUS-008.
Links and CV and gate regions
A bus board may split CV and gate into two regions (BUS-013). A bus board that splits CV and gate into regions shall mark on the board, and publish, which headers lie in each region (BUS-022). EIS recommends that a module publish whether it can transmit bus CV or gate. 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 board's supply input and its links to other bus boards are row wiring (6.1): M4 terminals that take ring terminals (PWR-005), on conductors of at most 5 mΩ (PWR-007), and its 0 V runs to the supply's 0 V terminal (GND-002).
EIS recommends that the 0 V copper from the board's 0 V terminal to its farthest header measure under 10 mΩ, a plane or a wide pour rather than a trace, and that the board take at least two 0 V links, on a second M4 terminal or a bus bar. The row wiring and the board carry the return current of every module on the board; a power ribbon carries the current of the modules on its sockets, through about 25 mΩ of 0 V at its 400 mm limit (BUS-015), its six 28 AWG conductors and the header contacts at each end.
A 0 V strip and bus bar
A bus board may carry a 0 V strip: a band of its outer-layer 0 V plane left bare of solder mask along the board's length, with plated holes at its mounting-hole positions, on which a 0 V bus bar is clamped by screws through the bar and the holes; the strip's end holes take M4, and a ring terminal under an M4 screw of the strip or the bar is a 0 V terminal of the board (BUS-021). The bar presses on the copper itself, with nothing between them but the finish, so the board's 0 V is the bar's the whole length of the board; the strip's M4 holes, two rings on each, give a board in the middle of a chain the positions one stud cannot, its return and a link each way (6.3). Without a bar the strip's M4 holes are still the board's link points.
EIS recommends:
- a strip at least 6 mm wide the full length of the board, on the plane's layer, finished as the board's pads are, beside the mounting holes rather than over them, with a plated M3 hole at every mounting-hole position, M4 at the two ends, and one more M4 position near the middle of a board longer than 42 HP;
- a copper bar, C110 or C101, no wider than the strip and under 0.5 mΩ end to end; the table gives the longest bar of each section. Brass is about four times the resistance and is not recommended. The bare strip alone, 70 µm (2 oz) copper 6 mm wide, is about 41 mΩ/m; the bar brings the board's 0 V under 1.5 mΩ/m, and under 1 mΩ/m from 6 × 3 mm up;
- a screw with a lock washer at every strip hole, torqued as the mounting screws are, so the joint stays under 0.1 mΩ for the life of the case; a ring terminal lands under the screw head, on the bar;
- the strip and the bar marked 0 V.
| Copper bar | Section | Resistance | Longest bar at 0.5 mΩ |
|---|---|---|---|
| 6 × 2 mm | 12.0 mm2 | 1.44 mΩ/m | 340 mm |
| 6 × 3 mm | 18.0 mm2 | 0.96 mΩ/m | 520 mm |
| 6.35 × 3.18 mm (1/4 × 1/8 in) | 20.2 mm2 | 0.85 mΩ/m | 580 mm |
| 6 × 4 mm | 24.0 mm2 | 0.72 mΩ/m | 690 mm |
| 6 × 5 mm | 30.0 mm2 | 0.57 mΩ/m | 870 mm |
A set of bus bars, or a flying bus of row wiring, that feeds modules through cables ending in power headers or ribbon sockets is a bus board for EIS: it meets this clause and 6.4, with its M4 positions as its terminals.
Distribution on a bus board
A bus board may also distribute a DC rail or the video sync behind the panel; it then meets the rules of what it carries: the PWR-DC12 outlet rules of 8.1, or the video sync ports of 8.2 (BUS-020).
- Its PWR-DC12 outlets are those of 8.1, each protected (DC-009, DC-012).
- As a video sync distributor, it takes the video sync at a COAX-IN and gives each module a buffered copy at a COAX-THRU (8.2).
The claim line is EIS draft, bus board whatever it carries (4.2), and the claim file lists the outlets and ports. A board that carries no bus at all, only a DC rail or the video sync tree, is still a bus board, with no PWR-BUS outlets. Everything on the board shares its one 0 V (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. A longer or thinner ribbon drops more at the header limit, and the drop comes off the module's rails: at 28 AWG (0.21 Ω/m), a 400 mm ribbon carrying 1.5 A on a rail drops about 65 mV in that rail's two conductors and about 30 mV more in their contacts at the two ends, about 95 mV; the 0 V return, six conductors and their contacts, about 25 mΩ, adds about 40 mV per 1.5 A it returns, up to about 75 mV with +12 V and +5 V both at the limit. 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.