24 Feb
2007
24 Feb
'07
9:17 p.m.
On 25 Feb 2007 at 0:19, Tony Duell wrote:
The original IBM Printer card (the printer-only one
used in the PC and
XT), but not the MDA adapter (which also included a printer port, of
course) have 3 little solder pads in a a line at 0.1" space. 2 of them
are linked by a trace. If you cut this trace and solder a 3-pin header in
place, you can fit a jumper link. In one position the card is a normal
output-only card (the OE/ line of the '374 that drives the data lines is
grounded), in the other potiion it's bidirectional (the OE/ line is
linked to the noprmally unused output of the '174 that is the control
output port). This is, AFRAK, not shown on the schematic in the techref
for some odd reason.
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
its end of useful life (superceded by a Herc Plus card) without
getting into any problems with anyone using the heretofore-unused bit
5 (IIRC) in the output control register. It seems that just about
everyone assumes that said bit should be zero.
Cheers,
Chuck