Re: Parallel port hardware


"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


.

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 ...
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  • Re: Parallel port hardware
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    ... 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)