25 Feb
2007
25 Feb
'07
12:35 p.m.
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