4 Jul
2006
4 Jul
'06
2:45 p.m.
It's also curious that the 6-bit latch (LS174)
used for some of the
handshaking pins has a free bit. I (and doubtless many others) found that
it was a simple matter to lift the tristate enable line and tie it to the
unused bit on the latch, giving full controllable bidirectional use of the
port. It's interesting that the PS/2 version of the parallel port did
exactly this; the bit used being identical in position and sense to the
original parallel port hack (probably a coincidence).
On the original IBM Printer Adapter PCB (although not on the MDA card,
which included a printer port) and some clones, the OR/ pin of the '374
is linked to ground by a track that has a couple of adjancent vias.
Another via nearby goes to the appropriate output of the '174. It's a
simple matter to cut the first track between the vias and solder a jumper
wire to link it to the via connected to thr '174.
I am not sure if this is shown on the schematic or not. It's certainly
very obvious if you look at the PCB.
Regardless, it's pretty obvious that the
"bidirectionality) of the parallel
interface was intended for self-diagnostic use more than anything.
I once saw a clone multi-i/o card that cheated with this. There weren't
enough pins on the ASIC to give the printer data port, so there was an
external 374, inputs from the system data bus, outputs to the printer
connector, and clock input from a pin on the ASIC. No buffer to read back
the printer lines, though. No, there was another latch hidden inside the
ASIC, which could be read back. It kept the BIOS routine happy, but
didn't actually check the printer port was working correctly.
If one needed +5, one could steal it by using a DC
boost converter from an
A boost converter cannot increase power. So if you, say, double the
voltage (e.g. from a TTL output sitting at about 2.5V up to 5V), you
halve the available current (down to <1mA?).
-tony