For What Do You Think The Classic II ROM Expansion Was Intended?

[Paralel]

I've been thinking about this for a while. It's inclusion in the design of the Classic II was a strange choice on the design team's part. No other Mac has a ROM expansion of which I am aware. Making a ROM drive requires making a change to the system ROM itself, so that doesn't appear to be what they were thinking. Although, do you think they changed course at one point and changed their mind, so they never included the ROM drive capabilities in the ROM intentionally? But if they changed their mind on the ROM expansion, why continue to include the option on the motherboard? They could have easily eliminated the jumper on the motherboard by just not putting the header on the board, not documented it, and left it as an undocumented feature.

I haven't been able to come to any firm conclusions on this. I'm hoping you guys will be able to provide some insight or theories that I haven't considered. Ideally I'd like to pick the brain of the person who was responsible for this feature in the first place, but that seems unlikely, especially since I don't even know who that would be.

Having been the driving force behind the creation of the ROM expansion card for the Classic II, and contributor to the design of the only card that has ever taken advantage of that feature, this has been hanging around in the back of my mind for awhile.

 

[joshc]

This is explained in the developer notes for the Classic II. I've copied the pages below.

FPU/ROM connector
The main logic board on the Macintosh Classic II computer includes
a 50-pin connector referred to as the FPU/ROM connector. This
feature allows third-party developers to design a card that can
provide a 68882 FPU coprocessor or additional ROM. All necessary
signals including address, data, FPU select, on-card ROM enable,
read/write, data strobe acknowledge, reset, and the system clock are
provided to the FPU/ROM connector. Another signal, ROM select, is
used in conjunction with a jumper on J9 to control the ROM
configuration. Two ROM configurations are possible:

• If the jumper is not connected (default condition) and the ROM
select signal is high, you can have 3 MB of additional ROM on the
FPU/ROM card but only 1 MB of main ROM socketed to the main
logic board.

• If you connect the jumper and the ROM select signal is low, you
can have only 2 MB of additional ROM on the FPU/ROM card and
2 MB of main ROM socketed to the main logic board.

♦ Important Apple strongly discourages the development of a
third-party FPU/ROM card for any type of internal
expansion other than a 68882 FPU coprocessor or
additional ROM. This includes any internal hardware
modifications such as clipping on to the main
processor.

Following are several reasons why you
should not use the FPU/ROM card for other types of
internal development:

• The power budget for the Macintosh Classic II
computer does not allow any margin for additional
internal devices.

• The Macintosh Classic II computer’s cooling fan has
insufficient capacity to dissipate the excess heat
that may result.

• Electromagnetic interference (EMI) testing did not
take into account the possibility of additional
internal devices or the fact that antennae are created
whenever additional external cabling is added.


28 Macintosh Classic II Developer Note

• Any additional load on the Macintosh Classic II
computer’s data lines could result in noise leading to
data errors and unreliable software.

• The address space assigned to the FPU/ROM
connector was originally envisioned to support
additional ROM therefore, it will support only read
access.

The development of an FPU/ROM card for any type
of internal expansion other than an FPU or a ROM
will not be supported by
Apple Developer Technical Support (DTS) and may
void Apple’s customer warranty.

 

[Androda]

No other Mac has a ROM expansion of which I am aware.

The Mac Portable has a ROM slot, and many Macs come with ROM SIMM sockets. The Mac Portable one was used for a backlit screen upgrade (rather a far cry from "ROM").

ROM sockets for SIMMs were probably left over as remnants from debugging with burnable prototype SIMMs, or for possible bug fixes if large issues appeared later on.

 

[Paralel]

The Mac Portable has a ROM slot, and many Macs come with ROM SIMM sockets. The Mac Portable one was used for a backlit screen upgrade (rather a far cry from "ROM").

ROM sockets for SIMMs were probably left over as remnants from debugging with burnable prototype SIMMs, or for possible bug fixes if large issues appeared later on.

I wasn't aware of the ROM slot on the Mac Portable. I've never been adventurous enough to want to own one.

This is explained in the developer notes for the Classic II. I've copied the pages below.

I've read the note, but for what did they think it would be used? Who would need 3 MB of ROM space for anything involving a Mac, especially a Classic II with its terribly limited hardware which has no intended expansion potential beyond an FPU? Besides the Portable, this is the only other Mac that has an intentional ROM expansion space, and not a small one at that. I have a hard time coming up with for what someone would use 3 MB of additional ROM space given the hardware at hand.

 

[joshc]

Besides the Portable, this is the only other Mac that has an intentional ROM expansion space, and not a small one at that

Lots of Macs had ROM slots - though there were seldom ROM upgrades available, that's a more recent thing.

From the dev notes it's clear that the main use was intended as an FPU upgrade slot, and there were upgrades from at least 3 manufacturers that I know of back then who provided FPU card upgrades.

The Classic had a bootable ROM built-in, so it could partly be a hangover from that and something they were planning to introduce.

ROM disks are useful - a faster way to launch applications so even on a Classic II a 3MB ROM disk would be very handy actually.

 

[Trash80toG4]

Does the Classic II boot from ROM as does the Classic?

If not, ROM on that interface might be a great place for a customized startup System, data a/o a few system maintenance applications.

 

[Paralel]

Does the Classic II boot from ROM as does the Classic?

If not, ROM on that interface might be a great place for a customized startup System, data a/o a few system maintenance applications.

Nope, it lacks the eDisk driver that the Classic has.