OK, now we're talking
First of all: please
cover that EPROM quartz window! We don't want that
valuable code erased by ambient UV light...
I shouldn't be writing this (nor even lurking the 68kmla forums) becasue I'm on the last days of a
terrible fortnight at work... but I promise to get back to this
very interesting project as soon as possible -- not only for the convenient use of popular PS/2 keyboards, but because it may shed some light on the
obscure ADB protocol...
Let's go with the ICs: the
brain of this device is the Micro Controller,
MC68L11D0 -- this is one of the many variants of the popular MC68HC11, with the following differences:
-Works on supply voltages as low as
3 V (although 5 V is fine with it)
-Has
no built-in ROM (the
-D0 part)
Like many other MCUs, this family may work, among others, in two modes:
single-chip (code is executed from
internal ROM, and most pins are dedicated to I/O functions) and
extended-multiplexed mode; where
data and address buses are externally accesible, and code is to be found on an
external (EP)ROM chip. This version lacking the internal ROM, obviously is configured to work on this last mode. From
my datasheets, it seems that this one can't go over
2 MHz.
Then there's the
TC232CPE, close to a bunch of electrolytic capacitors... as the name suggests, this is an equivalent of the popular
MAX232, just a converter from TTL/CMOS levels to the +12...-12 V required by th RS-232C serial port -- all from a single +5 V supply!
The EPROM is a
27C64-200DC: a mere
8 KiB EPROM in a rather slow (200 ns) version -- nothing out from the ordinary and shouldn't have that much code to investigate... anyway, this is
easily read/copied on the cheapest EPROM programmer (it is
socketed, isn't it?)
The remaining ICs are simple logic devices:
74LS05 (
open-collector inverters),
74HC00 (quad 2-input NAND gates) and featuring the
74HC573: 8-bit latch for the
multiplexed address/data bus.
Save for capacitors and resistor packs,
passive components are clear -- although the
exact frequency of the crystal might be critical for the interface timing.
The PCB seems to be a simple
2-layer design... shouldn't be that difficult to reverse-engineer -- as soon as I get some time, of course. I don't expect many surprises with the well known pinouts of these ICs, but another picture of the board
with the ICs off from their sockets could discard any doubts...
For the general public information: the Keyboard input on your device is the older
AT standard on
5-pin full-size DIN connector, but this is electrically compatible (protocol included) with the
PS/2 standard, just using a
6-pin Mini-DIN.
So many hacks, so little time... :b&w:
