Editing Processor Direct Slot

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[[Image:LC-PDS-Ethernet-Card.jpg|right|thumb|250px|LC PDS Ethernet card. PDS connector is at bottom left of photo. The card was mounted parallel to the main logic board, unlike most computer busses in which cards are inserted at right angles to the motherboard.]]
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 [[Personal computer|PC]]s. 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&nbsp;MHz speed.<ref name="lowendmac-iici">{{Cite web |title=Mac IIci |url=https://lowendmac.com/1989/mac-iici/ |access-date=2022-09-03 |website=lowendmac.com |archive-date=2022-09-03 |archive-url=https://web.archive.org/web/20220903213221/https://lowendmac.com/1989/mac-iici/ |url-status=live }}</ref>

== Overview ==
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 [[Macintosh Quadra 700|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.

== History ==

=== SE slot (Motorola 68000) ===
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]].<ref name="lowendmac-pds">{{Cite web |title=PDS: The Processor Direct Slot |url=https://lowendmac.com/2000/pds-the-processor-direct-slot/ |access-date=2022-09-03 |website=lowendmac.com |archive-date=2022-09-03 |archive-url=https://web.archive.org/web/20220903213239/https://lowendmac.com/2000/pds-the-processor-direct-slot/ |url-status=live }}</ref>

=== IIci cache slot (Motorola 68030) ===
The L2 [[Cache (computing)|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 IIvi|IIvi]], and [[Macintosh IIvx|IIvx]]. These allowed for 3rd party companies, such as [[DayStar Digital]], to develop processor upgrades that did not require the removal of the CPU.<ref name="lowendmac-iici" />
The IIci cache slot is different from the other PDS slot as it doesn't have provision for external connectors,<ref>DCDMF3, p. 529.</ref> and it does not include an interrupt request signal, thus limiting its use. Its pin configuration is also completely different from the one in most later MC68030-based non-LC Macintoshes.<ref>DCDMF3, p. 522.</ref>

=== IIsi, SE/30 slot (Motorola 68030) ===
The [[Macintosh SE/30]] included a PDS slot that uses the same 120-pin Euro-Din connector as the IIci, but a different pin configuration.<ref>DCDMF3, p. 318.</ref> The primary clock is the 16 MHz clock for the CPU. Unlike the IIci cache slot, it includes three interrupt request signals. There is also provision in the case for external connectors, so it was commonly used for graphic or network devices.
The [[Macintosh IIsi]] included a motherboard slot very similar to the SE/30,<ref>DCDMF3, p. 360.</ref> although clocked at 20 MHz like the onboard MC68030. This slot was not intended for direct use, Apple instead offered two adapter cards <ref>DCDMF3, p. 359.</ref> with a second connector mounted at a right angle (so the board would be parallel to the motherboard). One card provided a [[NuBus]] slot, while the other was essentially a pass-though configuration enabling a slot very similar to the IIsi, but with only one interrupt signal supported. Both cards also included an MC68882 FPU, which was not present on the IIsi motherboard.

=== IIfx PDS (Motorola 68030) ===
The [[Macintosh IIfx]], introduced in 1990, included a PDS that was almost identical to the SE/30 and IIsi,<ref>DCDMF3, p. 329.</ref> but not entirely. In particular, the clock signal is running at 20 MHz like the IIsi but the CPU is running at 40 MHz. The clock signal is also on a different pin from the IIsi and SE/30. Unlike other PDS, the connections to the processor signals are not direct, with intermediate buffering needed to handle the different clocks based on the physical address requested - some address would be accessed with the 20 MHz clock, and some other with the (not directly available on the slot) 40 MHz CPU clock ("fast slot space").<ref>DCDMF3, p. 355.</ref> The physical space available for the board was also different from the SE/30 and IIsi.

=== LC slot (Motorola 68020/68030) ===
The LC slot, introduced in [[1990]], began as a PDS for the [[Motorola 68020]] / [[68030]] processors in the [[Macintosh LC]] / [[Macintosh LC II|LC II]]. It only supported the asynchronous cycle of the MC68020, and did not connect all address signals, though it did connect all 32 data signals.<ref>DCDMF3, p. 305.</ref> The 96-pin Euro-DIN connector is similar to the SE slot connector, but the two are completely incompatible. 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. This configuration proved to be so popular for Apple's [[Performa]] line that later 68040 such as the [[Macintosh LC 475]] and [[PowerPC]]-based versions were essentially emulating the 68030 pin signals for the LC slots that they inherited.<ref name="lowendmac-pds" /><ref name="apple-maclciii">{{Cite tech report |date=2000 |title=Macintosh LC III Developer Note |url=http://developer.apple.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-68K_Desktop/Mac_LC_III.pdf |archive-url=https://web.archive.org/web/20040721012340/http://developer.apple.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-68K_Desktop/Mac_LC_III.pdf |archive-date=21 July 2004 |access-date=3 September 2022}}</ref>

=== Quadra PDS (Motorola 68040) ===
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 [[Macintosh Quadra 605|Quadra 605]] and [[Macintosh Quadra 630|Quadra 630]] were exceptions which used the full 114-pins version of the LC slot. Both allowed for [[PowerPC]] upgrades, including through the "Ready for PowerPC upgrade" program, using Apple's [[Macintosh Processor Upgrade Card]].<ref name="lowendmac-pds" />

=== Duo Dock connector (Motorola 68030 / NuBus) ===
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 unit]]s. Because of the complexity of larger [[Duo Dock]]s, [[NuBus]] was used to manage parts of the subsystems.<ref name="lowendmac-pds" />

=== Power Macintosh PDS (PowerPC 601) ===
First generation [[Power Macintosh]] systems with [[NuBus]] architecture, such as the [[Power Macintosh 6100|6100]], [[Power Macintosh 7100|7100]], and [[Power Macintosh 8100|8100]], included a PDS that was used for high-speed AV cards. It was later used by third-party manufacturers to support [[PowerPC G3]] and [[PowerPC G4|G4]] upgrades.

=== PowerPC daughtercard slot ===
High-end second generation [[Power Macintosh]] systems with [[Peripheral Component Interconnect|PCI]] architecture, such as the [[Power Macintosh 7500|7500]] through [[Power Macintosh 9600|9600]], had their [[PowerPC 601]] and [[PowerPC 604|604]] processors on daughtercards which could be swapped out for [[PowerPC G3]] and [[PowerPC G4|G4]] upgrades.

=== PowerPC cache slot ===
Mid-range [[Power Macintosh]] systems based on [[PowerPC 603e]] processors with [[Peripheral Component Interconnect|PCI]] architecture, such as the [[Power Macintosh 5400|5400]], [[Power Macintosh 5500|5500]], and [[Power Macintosh 6360|6360]] through [[Power Macintosh 6500|6500]], featured an L2 [[Cache (computing)|cache]] slot which provided direct access to the CPU. This allowed enterprising third-party manufacturers to build [[PowerPC G3]] upgrades for this slot.

More recent Macs have such high processor speeds that a PDS would not be practical and instead adopted [[PCIe]] and [[Thunderbolt (interface)]].<ref name="appleinsider-onecable">{{Cite web |title=One cable to rule them all: a look at Apple's retired connectors through the years |url=https://appleinsider.com/articles/18/07/04/a-brief-history-of-apples-retired-connectors-and-ports-over-the-years |access-date=2022-09-03 |website=AppleInsider |language=en |archive-date=2022-09-03 |archive-url=https://web.archive.org/web/20220903212758/https://appleinsider.com/articles/18/07/04/a-brief-history-of-apples-retired-connectors-and-ports-over-the-years |url-status=live }}</ref> The last implementation resembling such a slot by Apple is the processor tray connector in the 2009 to 2012 [[Mac Pro]]s. More recent models have had [[Intel]] Xeon processors socketed directly to the logic board.

== See also ==

* [[Peripheral Component Interconnect|PCI]]

== References ==
* [[File:CC-BY-SA icon.svg|50px]] Some of the text was originally copied from [https://apple.fandom.com/wiki/Processor_direct_slot Processor direct slot] at Apple Wiki, which is released under a [https://creativecommons.org/licenses/by-sa/3.0/ Creative Commons Attribution-Share Alike 3.0 (Unported) (CC-BY-SA 3.0) license].
* Apple Computer, Inc. (1992). "Designing Cards and Drivers for the Macintosh Family, Third Edition" (DCDMF3). ISBN 0-201-60855-3. 

{{reflist}}

==External links==
* [http://www.applefritter.com/taxonomy/term/122 Pictures of several PDS cards] at AppleFritter
* [https://web.archive.org/web/20080509063741/http://macfaq.org/hardware/cards.shtml#Q2.6.2 What kind of expansion or upgrade slots does my Mac have?] at Mac FAQ
* [https://apple.fandom.com/wiki/Processor_direct_slot Notes on PDS variations] at Apple Wiki

[[Category:Macintosh internals]]
[[Category:Motherboard expansion slot]]