Re: Parallel port hardware


"petrus" <iemand@xxxxxxxxxxxxx> wrote in message
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"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> schreef in bericht
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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)


Guesss you missed the point. That output *is* open collector but the read
back buffer has not been connected to that open collector output. See
below.

| |
.-. .-.
| | | |
| | | |
'|' '-'
| |
|\ 06 | |\06 | /INIT
---| >O---+---+---| >O---+------
|/ | |/
|
14/| | |
--------O< |--+ .-.
\| | |
| |
'-'
|\06 |
------------------| >O---+-+----
|/ | other control
|
14/| |
--------O< |---------------+
\|

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
Be aware that it is just one variant of the numerous schematics. Most
printerports I met had all read backs connected to the output pins.


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.


Can send you a scan of that printerport schematic I mentioned. Just to
give you an idea about the general appearence of the hardware. As for how
to handle them by the software, you have Beyond Logic. Can't see why it
should be less difficult to use a control line rather then a status line
for input. How many lines of what type do you need anyway? And what for?


That would work. Maybe send to alt.binaries?

The only problem is because the PGDx line for the device I'm connecting to
is bi-dir. There are times when it switches into send and sometimes into
recv. If I just connect a status line to the line then then control pin will
need to be disconnected. If not when the device goes into transmitt it will
short out if the control point is low impedence.

i.e.


basically what I have is


Vdd
|
R
|
PGDx ---0-----------+----0--- Control pin

Now if I hook a status pin for reading I have


Vdd
|
R
|
PGDx ---0--+--------+----0--- Control pin
|
|
Status Pin



If PGDx goes to Vdd then status pin reads Vdd but if control pin is low
impedence(i.e., grounded because its open collector) then I have a short. I
figure I can put a resistor inbetween like


Vdd
|
R
|
PGDx ---0--+---R1---+----0--- Control pin
|
|
Status Pin


And then I essentially can limit the current by R1 but I'm not sure how well
this will work.

Just seems easier to use the "bi-dir" capabilities of the control pin in the
first place and not have to worry about using a status pin.

I still have the probably of shorting if the control pin goes low-z so maybe
the status pin is easier.


There is a problem of speed though as if I have to read and write to the
ports to "set them up" then it slows down the transmission speed. Writing or
reading to a port takes about 3us on my computer. If I have to do it, say
3-4 times then I have significantly cut the speed down. I already do it 2-3
times because I have to have a clock that goes twice as fast. This is why if
I do everything on one register I can potentially combine some writes or
reads to maximize speed.

Thanks,
Jon


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 ;/


I like to use an extra printerport card while experimenting.


Thanks,
Jon


petrus bitbyter



.

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