Re: Parallel port hardware

petrus bitbyter pieterkraltlaatditweg@xxxxxxxxxxxxxxxxx posted to
sci.electronics.design:


"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> schreef in bericht
news:_QMMi.66$LD2.34@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Does anyone know how the open collector control port in the SPP is

+5
|
4.7 kohm
|
R____|________ TTL input/output
| out
|/
W---|
|\
|
Gnd



I can't seem to find anything that shows how the port is read from
and I'm trying to understand its behavior when reading based on it.
The above is
just what I think it probably is but I could be wrong. When
reading from the port W must be 0 and then R follows out. But this
isn't quite true because R seems to latch W to 1 and I have to
write W = 0 to allow R to follow out again.

Basically I'm trying to setup an input and output line into a
control port pin. I read that I have to use all the pins as either
input or output but not mixed? This is probably because I have to
reset the pin and I cannot do them individually? If this is the
case then I have to use a status port pin to read the line when it
is working as an input?... which makes it more complicated.

This is where I'm getting most of my info:

http://www.beyondlogic.org/spp/parallel.htm

and this is specifically where I'm having trouble:

"An external 4.7k resistor can be used to pull the pin high. I
wouldn't use anything lower, just in case you do have an internal
pull up resistor, as the external resistor would act in parallel
giving effectively, a lower value pull up resistor. When in high
impedance state the pin on the Parallel Port is high (+5v). When in
this state, your external device can pull the pin low and have the
control port change read a different value. This way the 4 pins of
the Control Port can be used for bi-directional data transfer.
However the Control Port must be set to xxxx0100 to be able to read
data, that is all pins to be +5v at the port so that you can pull
it down to GND (logic 0)."

Not sure why I have to really do all port pins for bi-direction.
(Obviously I can't read and write at the same time but I think I
can interleave them to get what I want)

Any ideas?

Thanks,
Jon


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.

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 :)

petrus bitbyter

Whatever IBM did was functionally very similar to one half an original
Intel 8255 without the ability to read the data port.

.

Relevant Pages

  • Re: Parallel port hardware
    ... schematic of a printerport that had no inputs but the status lines. ... The control line outputs used to be SN7406 ... I think the control port was always able to do ... If I just connect a status line to the line then then control pin ...
    (sci.electronics.design)
  • Re: Parallel port hardware
    ... The control line outputs used to be SN7406 ... I think the control port was always able to do ... Be aware that it is just one variant of the numerous schematics. ... If I just connect a status line to the line then then control pin will ...
    (sci.electronics.design)
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  • Re: OT: DOS programming EPP
    ... There seems to be *no* specifications or equivalent circuits for the parallel port as implemented on the ASICs used in modern PCs. ... it is completely unknown as to the maximum safe sink current to a logic low pin or the maximum source current from a logic high pin. ... The EPP page assumes you did. ... CONTROL bit 5 defaults low. ...
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  • Re: Parallel port hardware
    ... The control line outputs used to be SN7406 ... I think the control port was always able to do bidirectional ... Be aware that it is just one variant of the numerous schematics. ... you will have to check the printerport involved for every ...
    (sci.electronics.design)