Re: Parallel port hardware


I still want to lay my hands on the original IBM hardware manual. One of
the reasons is that parallel port. Nevertheless I saw several "compatibel"
schematics all with several differences. One thing is clear to me: The
output lines were never meant to do input. I have the schematic of a
printerport that had no inputs but the status lines. Others had "inputs"
on control- and data lines but they were only meant to read back the
status of that outputs. In the old days inputs used to be SN74LS14
inverting Schmidt triggered buffers, as were the read back inputs of the
control lines. The control line outputs used to be SN7406 inverting open
collector buffers. As the control lines are open collector, you can use
their read back inputs for real input when you drive that control lines
high... most of the time. I have a schematic in front of me in which the
read back of the /INIT control is taken from the input of the SN7406
buffer rather then from its output. So you will never read back the real
status of that printer pin. No need to say it will fail as an input as
well.


I'm not sure. I think the control port was always able to do bidirectional
because it was open collector(of course its true that not all parallel ports
were open collector on the control port but most are)

The schematic in the chapter
"Using The Parallel Port to Input 8 Bits"
will fail in this case.

So if you want to stay on the safe side, don't use output pins for input.
If you have to, you will have to check the printerport involved for every
(type of) computer. Once you'll have to do so, it will be worthwile to
check for other properties of the printerport at hand. Almost all but the
oldest computers have printerports that somehow can do bidictional data
transfer. If you have the choice, use EPP ports (or USB :)


Well, I only have one other option and that is to use a status line to read
in the data but then I have to "disengage" the output line from the data
line or use the open collector of the control port to somehow do it(which is
what I was going to do but since I can read the control port in the first
place theres no real reason to use the status line because it ends up making
it slower and I still have to disconnect the output line so screw everything
up).

Right now I'm just trying to program the thing for my computer and I think I
can do it with the control port only but it requires that I know how the
hardware port works and I really have no clue. Of course experimenting tells
me one thing but I'm not sure if I trust myself with it.

Using just the control port for what I want makes it very simple and
"elegant" compared to "hacking" it by mixing the status port and control
port. I guess the only way I'll know if it will work is to try it ;/ I
really hate doing that though cause its pretty risky ;/


Thanks,
Jon


.

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