Reading input to parallel port

[Peter Andersen]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground) I
get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something wrong?

Best regards.

 

[[email protected]]

Unconnected inputs are much more likley to read high than low; in some
logic families it's almost reliable.

Some of the parallel port signals are inverted between their electrical
levels on the bits accessable to software.

 

[Peter Andersen]

Unconnected inputs are much more likley to read high than low; in some
logic families it's almost reliable.

I suspect it has something to do with a pull-up resistor inside the port.
Is it possible to force the unconnected input-pin to logic 0 and logic 1
when a voltage source is connected (in this case +5V)?

 

[Roger Hamlett]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25
(ground) I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with
nothing connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

Undriven inputs, will 'float' to levels, depending on the logic family
involved. Historically, the TTL inputs used on printer ports, had a small
internal current, that drove them high. If you look at the equivalent
circuit for the original inputs, they have a resistance of about 4KR,
between the input, and the 5v supply rail. Latter chips using CMOS logic,
still emulate the original inputs. Note that the connection to pin 11, is
inverted, while the other default status inputs are non-inverted, which
allows a 'disconnected' status to be detected (this line is called
'Busy/Off Line' for this reason).

Best Wishes

 

[John - kd5yi]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25
(ground) I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with
nothing connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

http://www.lvr.com/files/ibmlpt.txt
http://www.lvr.com/jansfaq.htm

 

[Meindert Sprang]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground) I
get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something

wrong?

The printer port has rather strong pull-up resistors to VCC, so yes, it is
correct that you measure 5 V on the inputs and read all ones when left open.

Meindert

 

[Sjouke Burry]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground) I
get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something wrong?

Best regards.

Printer port input and outputs act like ttl , and the status pins
are normally high,because older printers could just use a switch
to pull the line to ground(no supply/electronics needed).

 

[Bob Monsen]

I suspect it has something to do with a pull-up resistor inside the port.
Is it possible to force the unconnected input-pin to logic 0 and logic 1
when a voltage source is connected (in this case +5V)?

Yes. You use a 'pull-down' resistor to gnd, and then a voltage source to
overcome that.

Pullups on PC parallel ports can be as small as 1k, so I'd go for a 220
ohm to ground. However, you can measure it easily by attaching a known
value resistor r to ground, and then Rpullup = r*(5/vmeasured - 1)

Then, choose a pull-down resistor that pulls it down enough to be seen as
a 0 by the port input driver.

---
Regards,
Bob Monsen

Even the greatest of creations start from small seeds.
- Unknown

 

[Chris Jones]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground)
I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

TTL inputs tend to float high when they are not connected. The newer chips
would be made to act like the old TTL chips too. This might explain what
you see.

 

[Andrew Holme]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25
(ground) I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with
nothing connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin
25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

It is working correctly. The inputs have pull-up resistors to 5V. They are
designed to be pulled down by open-collector or open-drain outputs e.g.


VCC
+
|
|
.-.
| | Pull-up
| | Resistor
'-'
|
Pin |
13 | |\
Apply .-----> >------o------| >O-
your | |/
5V |
here ___ |/

----|___|----| NPN

|>
10K |
|
===
GND

View in fixed-pitch font.
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[DaveM]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground)
I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

The status inputs on most IBM parallel ports are TTL (or derivatives
thereof) and are active low. This means that an unterminated device input
will float to a logical "1" unless specifically brought to a low state by
external circuitry. A TTL "1" is a voltage of 2.4V or greater (up to a max
of 5V).

Your port and your software are working properly.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[petrus bitbyter]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground)
I get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something
wrong?

Best regards.

Guess you'd better enlarge your knowledge about the parallel port interface.
One place - amongst others - is:
http://www.beyondlogic.org/
It lacks hardware details. Some more can be found at:
http://www.epanorama.net/circuits/parallel_output.html

Although pin description and register usage are well defined, hardware
details of the inner port are not very common. One of the reasons may be
that since the times of the first PC manufacturers used a variation of
components to implement its funcion. An average printerports hardware was
build like this:
- The datalines were TTL-outputs like LS273 or LS374. Some had smal
seriesresistors in the outputlines like 22E or 33E (E for Ohm). A capacitor
of about 470pF to GND was common although I often saw its space on the board
left empty.
- The controllines used open collector outputs like LS06 or LS07. The had
pullup resistors of let's say 4k7 and also capacitors to GND.
- The statuslines had LS14 Schmidtriggered inverters for input and low
(150E) pullup resistors. Place for capacitors was often available but left
empty.

It did not take long before special I/O chips took over the role of the
common TTL-components but the function was assumed to remain the same. I did
not always fit. I remember a parallelport chip build in CMOS that was blown
whenever the PC was switched off before the printer was. Nevertheless a
common parallel printerport will still be able to provide (about) the same
performance the old things did.

petrus bitbyter

 

[Peter Andersen]

Thanks for all the replies. They were all very helpful.

 

[scada]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground) I
get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something wrong?

Best regards.

No, it's working as expected. The status port is 5 bits, S7 is normally low
(inverted), S6, S5, S4, & S3 are all normally high. The open circuited byte
in binary would look like 01111111 , or 0x7F. The 3 least significant bits
(LSB S0-S2) are not there, they will allways be high. When you grounded Pin
13 (S4) you got 01101111 or, 0x6F. To change bit S7 you need to pull it high
to +5V (through a resistor - 10K should do it). To change bits S6-S3 you
pull them low (to ground, negative return). Good luck.

 

[Jasen Betts]

Hi,

I am trying to capture an input with a status pin on the standard IBM
parallel port.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something wrong?

you're doing it right.the pins are high by default, you need to connect the
pins to ground to change their status.

If you want it to be the other way round you can connect them to ground
using a 1K resistor and then they'd be low by default and applying +5v
would change thier state.


Bye.
Jasen

 

[Rich Grise]

I am trying to capture an input with a status pin on the standard IBM
parallel port.

I looks like all the status pins are set high default when there's no
external voltage source connected.
If I measure the voltage drop across pin 13 (status 4) and pin 25 (ground) I
get 5,1 V. Shouldn't it be zero?

I am using FreeBSD and if I read the value of the status pins with nothing
connected I get
Status: 0x7f
Nothing happens if I connect a 5 V voltage source to pin 13 and pin 25.

If I short circuit pin 13 and pin 25 I get
Status: 0x6f

Isn't it supposed to be the other way around or am I doing something wrong?

Actually, that sounds about right - they're pulled up internally, so you
have to pull them low to get a 0:
http://www.epanorama.net/documents/pc/parallelport_schematic.html

Have Fun!
Rich