6.4 The bus

The bus (PWR-BUS) is the power interface most Eurorack rows are built on: +12 V and −12 V, and optionally +5 V, made by a power supply, carried by bus boards and power ribbons, and taken by a module at its power header. A product that takes the bus at a power header owes the rules of a PWR-BUS entry, and one that gives it at outlets the rules of a PWR-BUS outlet; a product with neither, such as a passive module, owes nothing in this clause or the two that follow except BUS-011, which binds every module (6.4).

This clause defines the rails, the power header with its pinout, marking, and keying, 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 ring terminals on M4 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). Each rail within ±5 % (SUP-010); survives a short (SUP-004); at most 0.7 V reversed (SUP-005); overshoot ≤ 10 % (SUP-006). M4 terminals for ring terminals (PWR-005), each marked with its 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 1.5 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 socket (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 1.5 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). A power header on a module, a bus board, or a cable 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.

A module need not use the CV and gate lines; their levels and the rules for a module on them are in 6.6.

Marking and keying

The marked conductor of a power ribbon cable shall be −12 V, and on a module, a bus board, or a power supply it shall lie at the −12 V end of the header (BUS-004). A module, a bus board, and a power supply 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, and every header drawing uses it: pins 1 and 2 at the bottom, the gap centered in the right-hand wall. 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 −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 even odd pin 1, odd column, beside the key gap 10-pin header marked conductor on −12 V (BUS-004) The pinout follows BUS-003 and the key gap BUS-005. A module that uses neither +5 V nor bus CV or gate may use the 10-pin header (BUS-007).

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 Gate CV +5 V +12 V 0 V 0 V 0 V −12 V marked conductor on −12 V the key tab enters the gap marked conductor on gate the key tab meets the wall 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).

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). Like every power entry, it publishes its current on each rail (PWR-001).

A module or a power supply shall not draw more than 1.5 A on any rail through one power header (BUS-008), the charging of its decoupling at power-up excluded: each rail has two IDC contacts rated 1 A, and through a short ribbon they share the current up to about 2:1, so 1.5 A keeps the busier contact at its rating. 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 −12 V at pin 1, the ribbon's marked conductor (BUS-004); the board marks that end (BUS-019) 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 (BUS-008), about 1 A on each of its two IDC contacts; 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) and ties every 0 V pin to its 0 V (GND-001). Another header on the module does not mate a power ribbon, or survives one (BUS-011).

A module or a power supply shall tolerate reverse polarity at its power header without damage (BUS-009): a ribbon fitted backwards is the commonest field fault. Reversed at the module end it puts −12 V where +12 V belongs and the reverse, which a module survives with series diodes or a reverse-blocking regulator. Reversed at the bus end, a 16-pin ribbon also joins bus +12 V, +5 V, and CV through the module's 0 V pins, for every module on the board; the module cannot prevent that, and 6.2 recommends a supply that survives it. EIS covers only the pinout above and a reversed ribbon; any other voltage on these pins is outside EIS.

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 header of a different pitch or connector family, or a shrouded header too small for the socket, meets it by not mating; a 2.54 mm header a ribbon socket fits, shrouded or not, meets it only by surviving a ribbon either way round, since an unkeyed socket enters any shroud; Annex G covers expanders and links.

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 2 × 7 header mates a 2 × 5 socket as readily as a 2 × 5 header does, so the rule reaches any rear 2.54 mm header, such as an expander link (Annex G).

EIS · An expander header beside the power header power header, 2 × 5 expander, 2 × 5, same key a ribbon fits: survive it expander, 2 × 4 a ribbon does not fit expander, 2 × 6, unshrouded a ribbon fits: survive it 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 pitch or connector family, or a shrouded header too small for the socket, meets the first; protection on every pin meets the second.