Vector Imagery

[Tony Duell]

> 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