NuBus

NuBus is a 32-bit synchronous multimaster backplane bus originally developed at MIT (1978–1979) by Professor Steve Ward and colleagues as part of the NuMachine workstation project, then commercialised by Texas Instruments for its Explorer Lisp workstation in 1983. The bus was standardised as IEEE 1196-1987 (ANSI approved February 1988, withdrawn March 2000) and operates at 10 MHz, yielding a maximum burst throughput of 40 MB/s. Apple adopted NuBus as the expansion interface for the Macintosh II family (1987–1993), fitting six slots in the Mac II, IIx, and IIfx, and three slots in the IIcx and IIci. The physical connector is a 96-pin DIN 41612 (Euro-DIN / IEC 60603-2) three-row connector with 32 pins per row (rows A, B, C) on a 2.54 mm pitch; the card edge carries a male plug and the backplane carries the female receptacle.

Row A

Row B

Row C

Notes

• The physical connector is a 96-pin DIN 41612 (Euro-DIN / IEC 60603-2) three-row connector, 32 pins per row (A, B, C) on a 2.54 mm (0.1 inch) pitch. The card edge carries the male plug; the backplane (computer side) carries the female receptacle.
• Row B is the power and ground plane row in standard NuBus: pins B1 and B32 carry ±12 V, pins B4–B7 and B28–B29 carry +5 V, and pins B2–B3 and B12–B23 and B30–B31 are ground. NuBus-90 later repurposed Row B pins 8–11 and 24–27 for new control signals.
• Pins A2 and C2 are reserved no-connect in standard NuBus (IEEE 1196). The NuBus-90 proposal re-designates A2 as SB0 and C2 as SB1, forming a high-speed serial bus. Their listing as ‘-’ in the pin table is correct for IEEE 1196.
• Row B pins marked * (B8–B11) and ** (B24–B27) are reserved in standard NuBus. NuBus-90 assigns /TM2 (B8), /CM0 (B9), /CM1 (B10), /CM2 (B11) to the * group and /CLK2X (B24), STDBYPWR (B25), /CLK2XEN (B26), /CBUSY (B27) to the ** group.
• The bus clock (/CLK, Row C pin 32) runs at 10 MHz for standard NuBus. NuBus-90 adds /CLK2X at 20 MHz for card-to-card double-rate block transfers (~70 MB/s burst).
• Slot IDs are carried on the four /ID lines (ID0–ID3), enabling cards to self-identify during system startup without DIP switches or jumpers.
• Standard NuBus cards are backward compatible with NuBus-90 slots; a 10 MHz card operates normally in a Quadra backplane.

Signals

• /AD0–/AD31 — 32 multiplexed address/data lines; active-low, shared by all bus masters and targets. Address and data are time-multiplexed on the same lines.
• /CLK — 10 MHz synchronous bus clock; all NuBus transactions are clocked relative to this signal.
• /RESET — System reset; asserted by the bus master (host) to initialise all cards.
• /START — Indicates the start of a bus transaction; driven by the current bus master.
• /ACK — Acknowledge; driven by the addressed slave to confirm receipt of a transaction.
• /TM0, /TM1 — Transaction mode bits; together they encode the type and size of the current bus cycle (e.g. read, write, block transfer).
• /ARB0–/ARB3 — Four-wire fair arbitration lines; each potential bus master drives and monitors these lines to win bus mastership without a central arbiter.
• /RQST — Bus request; asserted by a card to request mastership of the bus.
• /NMRQ — Non-master request; used by cards that never act as bus master to signal attention to the host.
• /ID0–/ID3 — Slot-ID lines; hard-wired per slot on the backplane so each card can determine its own slot address at startup. Standard NuBus address space per slot: $FA000000–$FDFFFFFF; super-slot space: $A0000000–$DFFFFFFF.
• /SPV — Supervisor address; distinguishes supervisor-mode from user-mode accesses.
• /SP — Supervisor parity; provides parity protection for supervisor-mode transfers.
• /PFW — Power-fail warning; asserted by the power supply to alert cards of an impending power failure, allowing orderly shutdown.
• +5 V, +12 V, -12 V — DC power rails distributed across all three connector rows.
• GND — Signal and power ground; heavily distributed across rows A, B, and C for low-impedance returns.
• A2 (-) — Reserved no-connect in IEEE 1196 NuBus; re-designated SB0 (serial bus 0) in the NuBus-90 proposal.
• C2 (-) — Reserved no-connect in IEEE 1196 NuBus; re-designated SB1 (serial bus 1) in the NuBus-90 proposal.

References

• IEEE 1196-1987 — IEEE Standard for a Simple 32-Bit Backplane Bus: NuBus
• NuBus Specification, Document 2242825-0001 (Texas Instruments, 1983) — via Bitsavers
• Designing Cards and Drivers for the Macintosh Family, Third Edition — Apple Computer / Addison-Wesley, 1992.
• Guide to the Macintosh Family Hardware, Second Edition (Apple Computer / Addison-Wesley, 1990) — via Internet Archive

Category:Buses Connectors