All Macintoshes are capable of achieving the 230 kbps (kilobits per second) transfer rate, since this is the speed of LocalTalk. The problem is, unless a device uses the LocalTalk protocol, the port cannot stream that way and is limited to normal serial modes.

On a 16 MHz Mac II, the maximum speed in serial mode is probably around 20 kbps, even though the devices must talk at 56 kbps. Since the connection speed must be constant to this device, the machine pads data that is in the buffer, basically sending a bunch of null characters to pad the data stream to 56 kbps.

On faster machines with faster serial bus access, the buffers may be larger and more capable of sending closer to 56 kbps worth of data. This seems to be the convention for machines that break the 16 MHz barrier – most 68030s and higher.

Not specifically related, but around the same generation – when Apple’s started including FPU’s in the stock machine – the motherboards started supporting faster serial transfers again. There is no direct relationship, just kind of a threshold of embedded technology that allowed better things.

I’ve never quite looked at this in this light. I’m embarrassed that I’ve never noticed this before, but it’s absolutely correct. The 68882 is connected to the PDS bus (on the Mac IIsi, the 68882 literally connects to the PDS slot and routes out a NuBus slot to another card), while keeping the PDS portion for the FPU on the adapter. This magically pops a few things into my head that suddenly make a lot of sense. FPU insertable machines without PDS slots will have faster FPU access than machines with PDS slots, simply because the PDS functions aren’t serialized later on a slot.

Since QuickDraw can utilize the FPU to render into a printable image stream, I suspect that the spooling will take longer on a 68LC040. Transferring the final product shouldn’t be affected at all. The only thing you didn’t mention above is bus speed, which gets very critical at speeds 16 MHz or below – there isn’t much bandwidth left, after normal system operations, for background processes.

System 7.5.5 is very noisy, taking up 7-8 MHz of bus traffic even when idle. Switching the background printer in and out of this 50% traffic jam can boost utilization to the max, meaning there are object devices popping in and out of the data stream as their priority calls for it (your mouse will become hard to move and sluggish). At 20 MHz, on a full 32-bit bus (instead of the partials that 030’s had, with a few exceptions), 7.5.5 drops to 3-4 MHz of constant bandwidth utilization, simply because of the wider data bus.

I know I’m analogizing myself into a big hole and over simplifying a bunch of topics – those with the error correcting typewriters are free to loose the dogs on me….

Scott L. Barber <serker@earthling.net>

Pres/CEO, SERKER Worldwide, Inc.

Providing Hardware/Networking/Telecomm for 13 years

Scott L. Barber first posted this to Quadlist, the listserv for users of 68040-based Macs. It is reprinted with his permission.

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