18 Oct
2011
18 Oct
'11
1:56 p.m.
On Mon, 17 Oct 2011, Tony Duell wrote:
I didn't think any of the PC-family (ISA) IBM printer ports were
bidirecitonal 'out of the box', were they?
What i find curious is that the origianl printer interface card was
clearly designed to be trivially modifyable to be bidiectional. The mod
is always easy in concept -- disconnect the output enable pino of the
data latch from ground and connect it to the unused output of the control
port latch, and make sure the appropriate bus data line goes to the input of
this section of the latch.
But IBM made it easy. Firstly they did link up the data line to the input
of the unused section of the cotrol latch. Secondly, the track groudning
the ouptu enable pin of the data latch had a couple of vias on it, all
you had to do was cut it between those vias. And the 'unusued' output of
the control latch went to a via next to that. Sit was just a matter of
cutting the track and soldering in a short jumper.
What i find curious is that this is not mentioned in the TechRef.
-tony
Being pedantic about it, it appears you were
bit-banging serial I/O over
lines of the IBM printer interface, in which case I think it becomes a
serial port :-).
and LATER IBM "printer interface"s did not require ANY modifiaction for
use bi-directionally, in spite of that NOT being what IBM had "intended".