laser-lovers@uw-beaver (laser-lovers) (11/16/84)
From: allegra!utzoo!henry@uw-beaver.arpa
If anybody is interested, I have deciphered the circuitry inside the
LaserJet's font cartridges (at least, inside the TmsRmn cartridge).
It's pretty simple, just a bit of bus-interface stuff and two big ROMs.
The ROMs appear to be pinout-compatible with the 27256. The part numbers
on the ROMs are quite unrecognizable, probably internal numbers. They
are *not* socketed. There is no indication of how fast they have to be;
I've got the circuitry for the timing chain that supplies DTACK, but I
don't know the speed of the clock signal it uses.
There's space, pads, and decoding circuitry for two more ROMs, so the
actual capacity of the font cartridge would appear to be double what is
currently being exploited. There are some curious aspects to the decoding
circuitry, strongly suggesting that it was really intended for a larger
device containing still more ROMs (or something else).
I'm not motivated enough (yet) to unsolder the ROMs, and I won't be
able to rig up a LaserJet simulator to read the cartridge's contents
from its connector any time soon, so I don't know just what's in the
ROMs. If anyone out there is feeling ambitious, I'd be delighted to
swap a circuit diagram for a ROM dump.
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry