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PC port question

R

Randy Day

Jan 1, 1970
0
Before I plug this circuit into my PC, I'd like
to know if it's going to blow things up.

I'd like to use a 9v battery to power it,
rather than a 5v supply, since I'm using spare
parts for this. Can I use 9 volts and 1k
resistors, rather than the 5v and 330 ohms I've
seen in schematics? I know the current won't
blow the LEDs, but will the 9v cause problems
for the port circuitry? Are PC ports active
pull-up, or open-collector devices?

TIA

printer 1k LED
port ___
D0-|___|--|<--|
___ |
D1-|___|--|<--|
___ |
D2-|___|--|<--|-- +9v
___ |
D3-|___|--|<--|
___ |
D4-|___|--|<--|
___ |
etc.-|___|--|<--|

--- | |--
--- ---
- -
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
 
B

Baron

Jan 1, 1970
0
Randy said:
Before I plug this circuit into my PC, I'd like
to know if it's going to blow things up.

I'd like to use a 9v battery to power it,
rather than a 5v supply, since I'm using spare
parts for this. Can I use 9 volts and 1k
resistors, rather than the 5v and 330 ohms I've
seen in schematics? I know the current won't
blow the LEDs, but will the 9v cause problems
for the port circuitry? Are PC ports active
pull-up, or open-collector devices?

TIA

printer 1k LED
port ___
D0-|___|--|<--|
___ |
D1-|___|--|<--|
___ |
D2-|___|--|<--|-- +9v
___ |
D3-|___|--|<--|
___ |
D4-|___|--|<--|
___ |
etc.-|___|--|<--|

--- | |--
--- ---
- -
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Its a few years since I did any of this stuff, but my advice would be,
If you are going to use 9V, to use a buffer between the LED and port.
Individual transistors will work fine without risking damage to your
computer.
 
M

Michael Black

Jan 1, 1970
0
Baron said:
Its a few years since I did any of this stuff, but my advice would be,
If you are going to use 9V, to use a buffer between the LED and port.
Individual transistors will work fine without risking damage to your
computer.
If nothing else, the buffer protects the parallel port. So if there's
an accidental short on the output, the parallel port survives. It's
easier to put some another buffer IC in a socket than it is to fix
the parallel port. In the old days, it was merely tedious (you'd
have to desolder the dead IC), but nowadays it may not even be possible
(because the parallel port is part of a much larger IC that is
unavailable to the end user).

Of course, there is always the risk that the wiring to the buffer
IC is done wrong.

Michael
 
E

Eeyore

Jan 1, 1970
0
Randy said:
Before I plug this circuit into my PC, I'd like
to know if it's going to blow things up.

I'd like to use a 9v battery to power it,

Don't.

Graham
 
R

Randy Day

Jan 1, 1970
0
[snip
Of course, there is always the risk that the wiring to the buffer
IC is done wrong.

Rats. Back to the parts bins.

Thanks everyone.
 
G

Gareth

Jan 1, 1970
0
Randy said:
Before I plug this circuit into my PC, I'd like
to know if it's going to blow things up.

I'd like to use a 9v battery to power it,
rather than a 5v supply, since I'm using spare
parts for this. Can I use 9 volts and 1k
resistors, rather than the 5v and 330 ohms I've
seen in schematics? I know the current won't
blow the LEDs, but will the 9v cause problems
for the port circuitry? Are PC ports active
pull-up, or open-collector devices?

TIA

printer 1k LED
port ___
D0-|___|--|<--|
___ |
D1-|___|--|<--|
___ |
D2-|___|--|<--|-- +9v
___ |
D3-|___|--|<--|
___ |
D4-|___|--|<--|
___ |
etc.-|___|--|<--|

--- | |--
--- ---
- -
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Not sure if it will do any damage, but I'm sure it wont work (assuming
you want to switch the LEDs on and off?)

When the parallel port pin is low current will flow from the +9v though
the LED and the LED will be on. That bit should be OK.

When the parallel port pin is high, it will be 3.3V (unless you have a
very old PC). You then have 9V - 3.3V across the LED and resistor, so
current will still flow and the LED will still be on, though not quite
as bright. You also have a problem that this current is going the wrong
way (into the parallel port when the pin is high). This may do damage.

PC parallel ports are NOT open collector.

Are you saying that you want to use 1k resistors just because you don't
have 330 ohms? You could try the original schematic with 1k resistors
in series with the LED in place of 330 ohm. The LEDs wont be as bright,
but probably still visible. Another thing you could try is paralleling
up the resistors - 3 X 1k in parallel would give 333 Ohms.

Gareth

--
 
R

Randy Day

Jan 1, 1970
0
Gareth wrote:

[snip]
PC parallel ports are NOT open collector.

I didn't know if they were or not; I was kinda
hoping...
Are you saying that you want to use 1k resistors just because you don't
have 330 ohms? You could try the original schematic with 1k resistors
in series with the LED in place of 330 ohm. The LEDs wont be as bright,
but probably still visible. Another thing you could try is paralleling
up the resistors - 3 X 1k in parallel would give 333 Ohms.

That's what I'll be doing. I found an old 7805,
and I have lots of 1k resistors. Thanks.
 
B

Baron

Jan 1, 1970
0
Gareth said:
Not sure if it will do any damage, but I'm sure it wont work (assuming
you want to switch the LEDs on and off?)

When the parallel port pin is low current will flow from the +9v
though
the LED and the LED will be on. That bit should be OK.

When the parallel port pin is high, it will be 3.3V (unless you have a
very old PC). You then have 9V - 3.3V across the LED and resistor, so
current will still flow and the LED will still be on, though not quite
as bright. You also have a problem that this current is going the
wrong way (into the parallel port when the pin is high). This may do
damage.

I know its academic, but where did you get 3.3 volts from ?
 
J

Jamie

Jan 1, 1970
0
Baron said:
Gareth wrote:




I know its academic, but where did you get 3.3 volts from ?

The parallel port uses TTL compatible signals < 0.7v - > 4.8v
and where did you get those standards if may ask?
i think many people have forgotten what the real
standards are.

if memory serves, -12 = low, +12 high.
and the threshold points of
+2.5 = high, -2.5 = low.
anything between that is undefined.

or has my years of work failed me?
 
M

Michael Black

Jan 1, 1970
0
Jamie said:
if memory serves, -12 = low, +12 high.
and the threshold points of
+2.5 = high, -2.5 = low.
anything between that is undefined.

or has my years of work failed me?
Yes, they've failed you.

"High" and "Low" is in reference to the logic family, rather than
an absolute. so your 12V bits would never apply to the TTL family
that can never see more than 5v. (And given those voltages, it
seems more like you are thinking of RS-232 that does have something
like that.)

The logic family likewise defines the threshold point.

But, once this is all in place, the fact that there is a range of
voltages for a "high" and a "low" means that you wouldn't know
absolutely what a "high" out of a ttl gate would be. It just has
to be above the threshold, and it might vary from IC to IC (within
a certain small range) and from function to function (because the
internal circuitry varies and that affects the exact voltage when
the output is high).

Michael
 
J

Jamie

Jan 1, 1970
0
Michael said:
Yes, they've failed you.

"High" and "Low" is in reference to the logic family, rather than
an absolute. so your 12V bits would never apply to the TTL family
that can never see more than 5v. (And given those voltages, it
seems more like you are thinking of RS-232 that does have something
like that.)

The logic family likewise defines the threshold point.

But, once this is all in place, the fact that there is a range of
voltages for a "high" and a "low" means that you wouldn't know
absolutely what a "high" out of a ttl gate would be. It just has
to be above the threshold, and it might vary from IC to IC (within
a certain small range) and from function to function (because the
internal circuitry varies and that affects the exact voltage when
the output is high).

Michael
if you say, so, i was talking about the RS-232 port, not the
5 Volt logic family..

have it your way, i'll stick with what i do.
thanks.
 
T

Tim Auton

Jan 1, 1970
0
Jamie said:
Baron wrote: [snip]
The parallel port uses TTL compatible signals < 0.7v - > 4.8v
and where did you get those standards if may ask?
i think many people have forgotten what the real
standards are.

if memory serves, -12 = low, +12 high.
and the threshold points of
+2.5 = high, -2.5 = low.
anything between that is undefined.

or has my years of work failed me?

You almost described (inverted) RS-232. But not quite. Parallel ports
("Centronics", IEEE 1284 etc.) are a different beast all together.


Tim
 
B

Baron

Jan 1, 1970
0
Jamie said:
and where did you get those standards if may ask?
i think many people have forgotten what the real
standards are.

if memory serves, -12 = low, +12 high.
and the threshold points of
+2.5 = high, -2.5 = low.
anything between that is undefined.

or has my years of work failed me?
You are talking serial ports !
 
B

Baron

Jan 1, 1970
0
Jamie said:
if you say, so, i was talking about the RS-232 port, not the
5 Volt logic family..

have it your way, i'll stick with what i do.
thanks.

If you want to discuss serial ports then fine ! The OP was talking about
a Parallel port device !
 
B

Baron

Jan 1, 1970
0
Michael said:
Yes, they've failed you.

"High" and "Low" is in reference to the logic family, rather than
an absolute. so your 12V bits would never apply to the TTL family
that can never see more than 5v. (And given those voltages, it
seems more like you are thinking of RS-232 that does have something
like that.)

The logic family likewise defines the threshold point.

But, once this is all in place, the fact that there is a range of
voltages for a "high" and a "low" means that you wouldn't know
absolutely what a "high" out of a ttl gate would be. It just has
to be above the threshold, and it might vary from IC to IC (within
a certain small range) and from function to function (because the
internal circuitry varies and that affects the exact voltage when
the output is high).

Michael

Succinctly put !
 
S

Sjouke Burry

Jan 1, 1970
0
Jamie said:
and where did you get those standards if may ask?
i think many people have forgotten what the real
standards are.

if memory serves, -12 = low, +12 high.
and the threshold points of
+2.5 = high, -2.5 = low.
anything between that is undefined.

or has my years of work failed me?
You have been playing with serial ports to much :) :)
LPT port is ttl, with a pull up to Vcc minus one or two
diode voltages(between 1 and 2 volts).
 
J

Jamie

Jan 1, 1970
0
Sjouke said:
You have been playing with serial ports to much :) :)
LPT port is ttl, with a pull up to Vcc minus one or two
diode voltages(between 1 and 2 volts).
yeah well, at least i have something to play with.
:)
 
G

Gareth

Jan 1, 1970
0
Baron said:
I know its academic, but where did you get 3.3 volts from ?

Determined experimentally using a multimeter and a few PCs.


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