Processor Direct Slot

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.

[2] 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.

These allowed for 3rd party companies, such as DayStar Digital, to develop processor upgrades that did not require the removal of the CPU.

This slot was not intended for direct use, Apple instead offered two adapter cards [7] with a second connector mounted at a right angle (so the board would be parallel to the motherboard).

Latter models added 18 more pins through a notched in-line extension to support the full MC68030 bus (synchronous cycle, all address lines, clock both from the CPU and backward-compatible 16 MHz), while retaining backwards compatibility with older cards.

[2] The PowerBook Duo line was introduced in 1992 with a unique 68030-based connector that could allow the subnotebook logic board to communicate with an array of desktop docking systems, some of which could be upgraded with floating-point units.

[2] First generation Power Macintosh systems with NuBus architecture, such as the 6100, 7100, and 8100, included a PDS that was used for high-speed AV cards.

Mid-range Power Macintosh systems based on PowerPC 603e processors with PCI architecture, such as the 5400, 5500, and 6360 through 6500, featured an L2 cache slot which provided direct access to the CPU.

More recent Macs have such high processor speeds that a PDS would not be practical and instead adopted PCIe and Thunderbolt (interface).