In article <90qv0a$k6@nnrp1.deja.com>,
gabe@my-deja.com wrote:
> Ok.. as a follow up and possible future info for anyone who ever has
the
> same problems. The A105 (2k) OS system ROM is corrupted. I replaced
> it, and it works fine. Now I just need to make a copy or burn one for
> myself. This chips pinout looks non-standard, does anyone know if
there
> is an eprom I can substitute? I can't find a decent web-page with
info
> about the chips.
> Also.. I am using an MC6821 instead of the 6520 PIA chip. It seems to
> be working with my limited testing. Anyone know of any major problems
> with doing this?
> Does anyone even repair their old crap??? :))
> -Greg
> Sent via Deja.com
> Before you buy.
Since I don't have a 400 or that chip in my hand, I can't say for
certain, but Atari was very tricky when producing ROMs for their
products. I believe they did it for the express purpose of
keeping 'hackers' from releasing a better product than they did, but at
any rate, they would swap around the chip select signal for an address
pin and then make the chip enable pin 'positive enabled' instead of
negative as EPROMs are. These are custom chips available only in mass
quantities from a slew of manufacturers and are known in the industry
as 'maskable PROM's. They are configured and burned at the factory so
the little guy has no choice but to use an inverter device to invert
the chip select signal and do a little re-wiring to get the pins right
for an EPROM. Just to confuse matters more, the chips are labeled with
custom 'in house' numbers that the factory will _not_ discuss with you
or anyone else, so you can't even find out just what the pin out or
memory size is exactly. You have to guess and when you are right, it
will work. I have heard of desperate fellows cutting a window in the
top of the chip and erasing them with UV just like an EPROM, then
reprogramming them, but I won't stand by the method or claim it works
in any way. Best of luck on that project, but having said all the
above, the OS ROM of an XL/XE is pin for pin compatible with a 27128
EPROM. I guess Atari gave up on the convoluted ROM policy at least for
that one (or more?)instance. Are you sure your ROM is corrupt and not
suffering from dirty pin/socket disease? These are old machines and
they get that bug from time to time, you know. The system boards on
the 400/800 were famous for that disease as their contacts were tin
plated for the early machines. A decent schematic should allow you
to 'pinout' the chip and compare the results to a 2716(?) EPROM.
The MC68B21 PIA is pin for pin compatible with our 6520 PIA, the
MC68A21 is however, a 1 Mhz version and should not be used, since our
8bits are clocked at 1.7 Mhz. I don't know what to tell you if you are
missing the 'B' or the 'A'. 'B' = 2 Mhz version. MC = Motorola.
Hitachi made the HD68B21 which we can also use. Atari used it without
the Atari 'CO' numbering system on their last runs of XE boards.
I think its patently obvious that I've never even had the top off of
any of _my_ Atari machines, - repair??, what's that? :)
Sent via Deja.com
Before you buy.