Strange Mac 512k RAM expansion by Calmos
So I picked up an old Mac 512k "fat mac" the other day. I was tearing it down for some maintenance, but encountered problems because a logic board was interfering with parts of the case. Turns out it was not a stock board - it was a 3d-party RAM expansion board: 512k Calmos 68k overview My teardown here:
I can't find any info at all about it, other than the model (CD101D1-3). Even the manufacturer (Calmos) is a mystery. Is it GLaDOS' more mellow sister? Who knows.
Edit: Jeff points out in the comments that Calmos was a chip manufacturer. The chip itself is a SCSI controller. Regardless, this thing is fascinating in its hackishness, so I'm writing up this post to share the strangeness and to see if anyone knows anything about this.
Now, think "expansion card installation" for a moment. Thinking of card edge connectors? SIL headers, perhaps? Ribbon cables? All normal. Well, this thing is a bit more... hardcore. To install, you take some SIL pin headers, solder them directly onto the existing CPU's pins then press the expansion card onto those headers.
Yes, this Frankenstein has its own 68k. Could it be disabling the Mac's 68k to replace it with this monstrosity? Who knows. There's also this extra 2x13 header on the board that goes out to an extra DB-25 on the back of the Mac (you can see it just to the right of the expansion board's 68k on the below picture). Absolutely no idea what that does. Edit: It's a SCSI port.
Anyway, here's some pics of the thing installed on the Mac's mainboard.
So, has anyone seen this thing before? Were they popular? It does seem to work - my 512k is reporting 2 megs of ram.
Editor's note: I ported over the two comments from my old blog software.
Jeff Walther August 29th, 2012 - 08:53
That is most likely a NewLife/NewBridge brand upgrade for the Mac 512KE and 128KE (needs the 128K ROMs). I talked to a NewLife engineer back in the early 90s and yes, they put the host’s CPU to sleep and use a whole replacement 68000. It still runs at 8 MHz, though.
In a 512KE it will use the logic board RAM, so one can get to 4MB total with two 1MB SIMMs and six 256KB SIMMs. This was a very economical solution back when 1MB SIMMs were ~$100 and 256KB SIMMs were ~$5. Or one could go to 2.5MB of RAM with eight 256KB SIMMs.
The header and cable are a SCSI port expansion. That CALMOS chip is the not the brand of the board. CALMOS is apparently a chip manufacturer, who, in this case, licensed the 53C80 SCSI chip. So that CALMOS 53C80 is the SCSI controller which that header connects to.
Be careful moving the jumpers around. They have to be configured properly for the SIMMs installed. So if it’s working now, don’t change the jumpers. And if you change the RAM, expect to need to change the jumpers.
Jeff Walther August 29th, 2012 - 08:54
Oh, just noticed that the board does, indeed, say Calmos on the silk screening. Interesting. I wonder if they had some connection to NewLife. The only company I ever saw market a board like that was NewLife, but there was a dizzying variety of upgrades available back then.