Re: Parallel port hardware
- From: JosephKK <joseph_barrett@xxxxxxxxxxxxx>
- Date: Sat, 06 Oct 2007 02:08:43 GMT
Roger Hamlett rogerspamignored@xxxxxxxxxxxxxxxxxx posted to
sci.electronics.design:
petrus bitbyter wrote:
"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> schreef in berichtThe oldest 'hardware' IBM motherboard tech-ref, I have, is the one
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
for the original 'AT'. Have the BIOS ones for an XT, but can't see
the hardware ones.
However the parallel port, is described in the manual for the video
card (wasn't on the motherboard on these machines). The output
lines, use a 74LS374 latch (or 373, on the multi-io cards). The
output enable, is permanently grounded, making the data lines
'output only'. This was the 'SPP' port.
Devices that performed 'input' from these ports, used 'nibble wide'
4 bit transfers, using the four input status lines for the return
direction. The first IBM, to have bidirectional printer pins, was
the PS/2. This doesn't have the automatic latch cycle of the EPP
port, or the DMA abilities of the ECP port, but allowed the main
data pins to be read as well as written. This is the 'bidirectional'
port. Then you have the latter EPP, and ECP ports.
Most chips generating 'modern' printer ports,can be programmed to
emulate the SPP mode, but few emulate the simple bidirectional mode
of the PS/2.
There used to be a standard 'mod' for the old printer cards, of
cutting pin1 on the output driver chip (the output enable line), and
connecting it to a spare latch on an LS174 on the card, allowing the
direction to be switched.
The outputs on the parallel port, were _not_ 'open collector'
drives.
Best Wishes
Drat, I had misremembered. The less old stuff is nearly like an 8255.
.
- References:
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- From: Jon Slaughter
- Re: Parallel port hardware
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- Re: Parallel port hardware
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