Franklinstein
Well-known member
A quick tidbit regarding the LC PDS:
Originally designed for the LC and its 16-bit system bus, this connector was built to directly interface to the 68020 at 16MHz. The LC PDS also included audio and FPU present pins in addition to a handful of others to allow the use of cards with some sound capabilities (specifically the IIe card) and to install an FPU in a machine where a socket was not present on the logic board, an example of which is shown in the picture of the Radius Pivot video card above and also familiar to people with early LC PDS Ethernet cards.
The LC II kept the 16-bit system bus and clock speed, the only change being a move to the '030, but since they shared the same bus protocols, the LC PDS still operated exactly the same as in the LC. This is also true of the Color Classic, which was essentially a slightly redesigned LC II logic board in a compact case.
The LC III (and III+, Color Classic II, Performa 520, 550, and 275, all the same basic architecture) changed things a little: the system bus was now 32-bits wide, but thanks to the 68030's dynamic bus sizing feature, a legacy 16-bit LC PDS card can be installed with no problems. Because the system bus was now 32-bits wide, a small extension to the original LC PDS slot was added that provided the signals to expand the slot up to a full 32 bits. Now known as the LC III PDS, this extended slot became Apple's standard low-end expansion slot across their entire consumer line for about 4 years, with (most) cards originally designed for the LC able to be used in up to a Power Macintosh 53/63xx series machine. Of course, after the switch to the 68040, the LC PDS is no longer a true PDS: it resides on an '040-to-030 bridge chip of some sort, and depending on which computer it's in. These psuedo-PDS slots also ran at a fixed 16MHz whether they were 16 or 32-bits and regardless of the system bus speed of the computer. The original Comm Slot is basically an LC PDS in a different form factor, with both running at a fixed 16MHz off of the '030 bridge chip.
What I'm not entirely sure about is whether the LC/LC III PDS was always clocked at 16MHz, even in the LC III and III+ where the system bus is 25 or 33MHz respectively. ISTR that it was stated in some of the Apple dev material that it is always fixed at 16MHz, but I don't know how it's achieved: the LC III PDS is still directly connected to the processor's pins in the LC III and III+ with no buffers or arbitration devices in between.
Originally designed for the LC and its 16-bit system bus, this connector was built to directly interface to the 68020 at 16MHz. The LC PDS also included audio and FPU present pins in addition to a handful of others to allow the use of cards with some sound capabilities (specifically the IIe card) and to install an FPU in a machine where a socket was not present on the logic board, an example of which is shown in the picture of the Radius Pivot video card above and also familiar to people with early LC PDS Ethernet cards.
The LC II kept the 16-bit system bus and clock speed, the only change being a move to the '030, but since they shared the same bus protocols, the LC PDS still operated exactly the same as in the LC. This is also true of the Color Classic, which was essentially a slightly redesigned LC II logic board in a compact case.
The LC III (and III+, Color Classic II, Performa 520, 550, and 275, all the same basic architecture) changed things a little: the system bus was now 32-bits wide, but thanks to the 68030's dynamic bus sizing feature, a legacy 16-bit LC PDS card can be installed with no problems. Because the system bus was now 32-bits wide, a small extension to the original LC PDS slot was added that provided the signals to expand the slot up to a full 32 bits. Now known as the LC III PDS, this extended slot became Apple's standard low-end expansion slot across their entire consumer line for about 4 years, with (most) cards originally designed for the LC able to be used in up to a Power Macintosh 53/63xx series machine. Of course, after the switch to the 68040, the LC PDS is no longer a true PDS: it resides on an '040-to-030 bridge chip of some sort, and depending on which computer it's in. These psuedo-PDS slots also ran at a fixed 16MHz whether they were 16 or 32-bits and regardless of the system bus speed of the computer. The original Comm Slot is basically an LC PDS in a different form factor, with both running at a fixed 16MHz off of the '030 bridge chip.
What I'm not entirely sure about is whether the LC/LC III PDS was always clocked at 16MHz, even in the LC III and III+ where the system bus is 25 or 33MHz respectively. ISTR that it was stated in some of the Apple dev material that it is always fixed at 16MHz, but I don't know how it's achieved: the LC III PDS is still directly connected to the processor's pins in the LC III and III+ with no buffers or arbitration devices in between.