Прямой слот процессора Macintosh: история и особенности

A processor direct slot (PDS) is a slot incorporated into many older Macintosh models that allowed direct access to the signal pins of a CPU, similar to the functionality of a local bus in PCs. This would result in much higher speeds than having to go through a bus layer, such as NuBus, which typically ran at a slower 10 MHz speed.

Typically, if a machine had bus expansion slots it would feature multiple bus expansions slots. However, there was never more than one PDS slot, as rather than providing a sophisticated communication protocol with arbitration between different bits of hardware that might be trying to use the communication channel at the same time, the PDS slot, for the most part, just gave direct access to signal pins on the CPU, making it closer in nature to a local bus. Thus, PDS slots tended to be CPU specific, and therefore a card designed for the PDS slot in the Motorola 68030 based Macintosh SE/30, for example, would not work in the Motorola 68040 based Quadra 700. The one notable exception to this was the PDS design for the original Motorola 68020 based Macintosh LC. This was Apple's first attempt at a "low cost" Mac, and it was such a success that, when subsequent models replaced the CPU with a 68030, a 68040, and later a PowerPC processor, Apple found methods to keep the PDS slot compatible with the original LC, so that the same expansion cards would continue to work.

The SE "System Expansion" slot, introduced in the Macintosh SE in 1987, was the first processor direct slot, using a 96 pin Euro DIN connector to interface with the Motorola 68000 processor. This slot was also used in the Macintosh Portable.

The L2 cache slot of the Macintosh IIci, introduced in 1989, was a 32 bit version of a PDS which used a 120 pin Euro DIN connector to support the Motorola 68030 processor. This slot also appeared in the Macintosh IIsi, IIvi, and IIvx. These allowed for 3rd party companies, such as DayStar Digital, to develop processor upgrades that did not require the removal of the CPU. The Macintosh SE/30 also uses the same PDS configuration, but space inside the compact Mac case is a limiting factor.

The LC slot, introduced in 1990, began as a PDS for the Motorola 68020 / 68030 processors in the Macintosh LC / LC II, which used a slower, less expensive 16 bit data bus. The 96 pin Euro DIN connector in the early 16 bit models look like the SE slot connector, but are incompatible. Latter models added 18 more pins through a notched in line extension to support a 32 bit data bus, while retaining backwards compatibility with 16 bit cards. This configuration proved to be so popular for Apple's Performa line that later PowerPC based versions were essentially emulating the 68030 pin signals for the LC slots that they inherited.

The Macintosh Quadra series was introduced in 1991 with a new PDS for the Motorola 68040 processor which was in line with one of the NuBus slots. The Quadra 605 and Quadra 630 were exceptions which used a 32 bit version of the LC slot. Both allowed for PowerPC upgrades through the "Ready for PowerPC upgrade" program. The last implementation resembling such a slot by Apple is the processor tray connector in the 2009 to 2012 Mac Pros. More recent models have had Intel Xeon processors socketed directly to the logic board.