6.4 The bus

The bus is the power domain every Eurorack row has: +12 V and −12 V, and optionally +5 V, made by a power supply, carried by bus boards and power ribbons, and taken by each module at its power header. This clause defines the rails, the power header, its pinout, marking, and keying once, and what a module owes at its header; 6.5 adds the bus board and the ribbon, and 6.6 the CV and gate lines.

EIS · From the supply to the module: what each part owes power supply +12 V −12 V +5 V tabs or a keyed connector bus board power ribbon module draws from the bus Power supply Publishes each rail's rating (PWR-002) and its ripple and noise (SUP-001), below 150 mV (SUP-007). Survives a short (SUP-004); no reverse swing (SUP-005); overshoot ≤ 10 % (SUP-006). Keyed connector (SUP-002), or terminals marked with their rail (SUP-003). Bus board +12 V and −12 V, and optionally +5 V, at every header (BUS-001). Publishes its rail totals and the limit per header (PWR-002), at most 2 A per rail (BUS-012). A link joins CV to CV and gate to gate (BUS-014). Ribbon At most 400 mm, at least 28 AWG (BUS-015). Marked conductor on −12 V (BUS-004); keyed (BUS-005). Module Draws only ±12 V, and +5 V if it needs it (BUS-006). Publishes its maximum current on each rail (PWR-001). At most 2 A per rail through one header (BUS-008); never feeds a rail (PWR-004). Survives a reversed ribbon (BUS-009). The installer adds a module only while each rail's sum fits the bus board and the supply (ASM-001).

Rails

The bus shall provide +12 V and −12 V, and may provide +5 V, at every power header (BUS-001).

Header and pinout

A power header on a module or a bus board shall be a 2 × 5 or 2 × 8 pin header on a 2.54 mm grid, and a power cable shall end in IDC ribbon sockets that mate it (BUS-002). It shall follow this pinout (BUS-003):

Pins 10-pin header 16-pin header
1, 2 −12 V −12 V
3 to 8 0 V 0 V
9, 10 +12 V +12 V
11, 12 — +5 V
13, 14 — CV
15, 16 — Gate

Some community tables give a different order, with +12 V on pins 7 and 8 and +5 V on pins 15 and 16. A cable or bus board made to such a table puts +12 V on a module's 0 V pins.

On a 16-pin header, pins 13 and 14 carry one CV line and pins 15 and 16 one gate line. Using them is optional; their levels and the rules for a module on them are in 6.6.

Marking and keying

On a module, a bus board, and a power ribbon cable, the marked conductor shall be −12 V and shall point to the −12 V end of the header (BUS-004). A module and a bus board shall mark the −12 V end of each power header on the board (BUS-019), so that an unshrouded header can be oriented by eye at both ends.

A shrouded power header on a module or a bus board shall be keyed so that, viewed from the mating side with the −12 V end at the bottom, the key gap is on the right, and a keyed cable socket shall match it (BUS-005). The figure below is the reference view: pins 1 and 2 at the bottom, the gap centered in the right-hand wall; a drawing of a header shows the same view. Shrouded headers in the field are keyed both ways, so a keyed cable from a conforming product enters a conforming bus board and may not enter another.

EIS · Power header, 16-pin, from the mating side key gap on the right −12 V 1–2 0 V 3–4 0 V 5–6 0 V 7–8 +12 V 9–10 +5 V 11–12 CV 13–14 Gate 15–16 odd even 10-pin header marked conductor = −12 V (BUS-004) Viewed looking into the pins with the −12 V end at the bottom: the shroud key gap is on the right (BUS-005). Pinout: BUS-003.

EIS · The marked conductor and the shroud key Socket the right way round Socket reversed Gate CV +5 V +12 V 0 V 0 V 0 V −12 V the tab enters the gap Gate CV +5 V +12 V 0 V 0 V 0 V −12 V the tab meets the wall marked conductor on −12 V marked conductor on gate Seen from the mating side with the −12 V end at the bottom, the shroud's key gap is on the right (BUS-005). On a module, a bus board, and a ribbon, the marked conductor is −12 V and points to the −12 V end of the header (BUS-004). A cable that comes with a module is keyed, so it enters a shrouded header only the right way round (BUS-010). An unshrouded header cannot stop a reversed socket, so every module also survives one (BUS-009).

Any voltage on these pins other than the pinout and a reversed ribbon is outside EIS.

A module on the bus

A module powered from the bus shall draw only +12 V and −12 V, and may also draw +5 V (BUS-006). A module that uses neither +5 V nor bus CV or gate may use the 10-pin header (BUS-007). It publishes its current on each rail as every power entry does (PWR-001).

A module shall not draw more than 2 A on any rail through one power header (BUS-008): an IDC contact is rated about 1 A, and each rail has two. A module that needs more uses a second header or a DC entry.

EIS · Module power entry: what a module may draw, and through which header 1 −12 V 2 −12 V 3 0 V 4 0 V 5 0 V 6 0 V 7 0 V 8 0 V 9 +12 V 10 +12 V 10-pin no +5 V, CV, or gate 1 −12 V 2 −12 V 3 0 V 4 0 V 5 0 V 6 0 V 7 0 V 8 0 V 9 +12 V 10 +12 V 11 +5 V 12 +5 V 13 CV 14 CV 15 gate 16 gate 16-pin the full bus pin 1 at the marked conductor Draws only +12 V and −12 V, and may draw +5 V (BUS-006). The 10-pin header is enough when the module uses neither +5 V nor bus CV or gate (BUS-007). At most 2 A on any rail through one header: two IDC contacts of about 1 A each (BUS-008). A module that needs more takes a second header or a DC entry. Never sources current into a bus rail (PWR-004). Survives a reversed ribbon (BUS-009); its every 0 V pin is tied to its 0 V (GND-001). Another header on the module does not mate a power ribbon, or survives one (BUS-011).

A module shall tolerate reverse polarity at its power header without damage (BUS-009): a ribbon fitted backwards is the commonest field fault. A bus cable supplied with a module shall be keyed so that it enters a shrouded header only with the marked conductor on −12 V (BUS-010). On a bus board whose shroud is keyed the other way, outside EIS, the installer fits an unkeyed cable with the marked conductor on −12 V.

Other headers

A header on a module that is not its power header shall either not mate a Eurorack power ribbon socket or tolerate one fitted to it, either way round, without damage (BUS-011). A 2 × 7 header mates a 2 × 5 socket as readily as a 2 × 5 header does, so a module that takes its video sync on a rear ribbon header is one this rule reaches. Expander and link headers are often 2 × 5 or 2 × 8 pin headers on the same grid, and a power ribbon fitted to one puts ±12 V on its signal pins. A shrouded header keyed differently, a different pin count, or a different connector family meets this; Annex N covers expanders and links.

EIS · An expander header beside the power header power header, 2 × 5 expander, 2 × 5, same key a power ribbon fits expander, 2 × 4 a power ribbon does not fit expander, 2 × 6, unshrouded fits only if it survives ±12 V A header that is not the power header either does not mate a power ribbon socket or survives one fitted to it, either way round (BUS-011). A different pin count, a different key, or another connector family meets the first; protection on every pin meets the second. Annex N lists expanders and digital links on the port list as unclassified, named by what they connect to.

Wiring

EIS recommends blue for −12 V, black for 0 V, and red for +12 V in the distribution wiring of bus boards and supplies, and names no color for +5 V. Chassis ground is in 6.3.