> The MDA port can also be reconfigured as bidirectional,
but IIRC,
> it's a cut trace and a jumper. I did that with one and
ran it until
Sure. As can a lot of the clone printer cards. The mod is to
disconnect
the OE/ pin of the data latch ('374) from ground and to
connect it to the
bit 5 (?) output of the control port (nearly always a '174,
and nearly
always the input of this section is already wired to the D5
line from the
ISA bus).
It's just that the IBM printer (only) card had that postion
for the 3-pin
jumper header, and one of the traces going to those solder
pads had no
other function other than to make the port bidirectional.
Incidentally, I once saw a clone dual serial and one
parallel port card
where there was a 40 pin ASIC that implemented the first
serial port and
most of the printer port (the control/status lines, for
example). But the
printer data port was a separate '374 on the board (D inputs
from the
data bus, Q outputs to the DB25 connector), but with no way
to read it
back. It was clocked by a pin on the ASIC. Writing to that
port address
latched the data into the 374 (and thus made it available to
the
printer) and latched the data in a register inside the ASIC.
Reading that
port address read the register inside the ASIC.
Therefore it would pass the standard printer tests, but it
couldn't
detect a failure of the output latch, or a short in the
cable, or... And
of course it couldn't be made bidirectional
That card ended up in somebody else's machine...
-tony
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