NeXTbus

NeXTbus is a 32-bit parallel computer bus that was derived from NuBus, which was originally developed at the Massachusetts Institute of Technology and standardized by the Institute of Electrical and Electronics Engineers in 1987.

History
The first implementation of NuBus dates back to the 1970s with Western Digital's NuMachine workstation project, and Lisp Machines' LMI Lambda. The NuMachine and Lisp projects were acquired by Texas Instruments in 1983. The NuBus technology was then incorporated their own TI Explorer family of Lisp computers.

NuBus was also used as the main expansion bus by Apple Computer and NeXT from the late 1980s to the mid 1990s. It is no longer widely used outside of the embedded market and was withdrawn by IEEE on March 6, 2000.

Implementations
The first NuBus standard, also known as NuBus 87, was approved on June 11, 1987 as IEEE 1196-1987. This version used a standard 96-pin three-row Euro-DIN connector, running the system on a 10 MHz clock for a maximum burst throughput of 40 MB/s and average speeds of 10 to 20 MB/s. The first Apple model to use this implementation was the Macintosh II. However, it also implemented a 5 V trickle charge for detecting modem signals while powered down that was not part of the approved spec.

A proposed revision, NuBus 90, also known as IEEE P1196-R1990, increased the clock rate to 20 MHz, with burst throughput increasing to about 70 MB/s, and averaging about 30 MB/s. It also implemented a +5 V trickle charge for detecting modem signals while powered down. However, this spec was not formally approved as of November 12, 1991. The Macintosh Quadra series used a partial implementation in which NuBus 90 cards could communicate with one another at 20 MHz, but still used an older controller that communicated with the motherboard itself at 10 MHz. The Quadra AV and early Power Macintosh series used the full NuBus 90 implementation. By the time 2nd generation Power Macintosh models were released, Apple had switched to the PCI standard.

NeXT developed its own variant of the standard called NeXTbus, which used a different physical PCB layout. The bus was clocked at 25 MHz, with burst throughput up to 100 MB/s. Communication between NeXTbus cards required a NeXTbus Interface Chip (NBIC), which was an option in the original NeXT Computer and included with all 68040-based NeXTcubes.