🚀 go-pugleaf

In <15568@ucsd.Edu> brian@ucsd.Edu (Brian Kantor) writes:

>The IBM technical reference manual shows an LS374 in there, which is an
>8-bit latch.  The data lines are latched, but there's an LS244 reading
>them back in when you read from the port so that you can perform a
>self-test by reading back the byte you had just output.

>Thus you can make the port bi-directional if you want.  Just chop the
>wire from the latch's output enable pin, and run it out to a spare pin
>on the connector.  The external device would control the direction.

>There are other strobe and control lines, some in each direction.  Best
>you check a schematic if you're planning to hack it.

This subject has come up in the computing conferences, and one little
detail came up that's worth mentioning here.  Many "IBM-compatible"
machines differ from the PC in various minor ways (even IBM's own
laptop machine was notorious for this) and the printer port is an
obvious place to cut corners -- most printers don't really *need* a
bi-directional channel, do they?  The upshot is that one shouldn't
assume that a given clone's parallel port's bidirectional, and (say
some pundits) you can even damage some machines by trying to use them
that way.

I should emphasize that I'm not qualified to defend or even amplify
the above.
-- 

ergo@netcom.uucp			Isaac Rabinovitch
atina!pyramid!apple!netcom!ergo		Silicon Valley, CA
uunet!mimsy!ames!claris!netcom!ergo

	"I hate quotations.  Tell me what you know!"
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