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
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...