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'[PIC:] safely driving a pic input from a PC Parall'
2003\10\24@174008 by James Newton, webhost

face picon face
I've been thinking about how things get connected between PC's and PIC's
lately and overall what I can see is:
USB: Nice! But not the lowest possible cost and not supported on some old
PC's Also, requires some driver development on the PC side... not hard, but
still.

RS232: Nice... But you can run out of ports, some current laptops don't even
have serial ports (!) and you have to do all the darn configuration on the
PC to make sure all the settings are right. Has anyone been able to get a
USB to RS232 adapter to work right? I can never get the PC application to
see the port.

Parallel: Sigh... NT and up driver problems (IF you don't simulate a printer
with the PIC) but there are solutions for that. One the one port usually and
I've seen laptops without parallel ports. USB to parallel adapters are
really shaky... has anyone used one to talk to anything other than a
printer? The two I've tried don't show up as LPT ports on the PC and so old
applications don't see them.

Of all the above, probably the most reliable, simple, and cheapest for us
little guys is the parallel port (this is IMHO and I'm sure the point can be
argued 20 ways from Sunday).

So, if I want to put out something that people can setup from a PC, a
temporary connection via the parallel port is probably best. Now the
question is how do I keep people from bread boarding something that connects
via my device that will blow up the PC (as well as me probably) when they
screw up?

I went on a hunt for parallel port isolation and, not being the most
brilliant analog electronics engineer (ok, so I'm a complete moron in that
area) I really don't seem to understand most of what I found.

I understand that the port can sink 24mA and only source 2.6mA so obviously
its a good idea to drive your loads from the port, through the load, then to
+5v rather than ground.

Safe current limiting can be had with a series resistor of 1.6k or more
right? (4.2V / 2.6mA) But that doesn't leave much drive... so you can add a
driver, but wouldn't it be better to prevent the pin from sourcing anything
by putting a diode long with the resistor in series and then reducing the
resistor to 175 or more ohms (4.2V / 24mA)?


                      +5
                       |
                       /
                       \
                       /
      220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>
_|_/
//_\ 5.1V
 |
GND

A pull-up resistor is added to the output to drive TTL or other loads and a
Zener could be added to ground to prevent over voltage or driving the pin in
reverse. That seems to me to be a nice little, very low cost, easy to work
with, circuit to provide some basic protection for kids or students to tack
things onto.

Not wanting to assume I have a clue, I searched the internet for other
solutions. Tomi Engdahl says:

"The following circuit uses two 1N4148 diodes to protect parallel port
against higher than +5V signals and also against wrong polarity signals
(power on the circuit is accidentally at wrong polarity. "

         Diode
         1N4148  4.7K
parallel  >-|>|-+--\/\/\/--etc...
port data       |
pin             +-|<|-+
           1N4148    |
parallel  >-----------+
port ground           |
                   Ground

"Adding even more safety idea: Replace the 1N4148 diode connected to ground
with 5.1V Zener diode. That diode will then protect against over-voltage
spikes and negative voltage at the same time. "

Ok, he has his series diode in the exact opposite direction. Could somebody
please tell me if I'm just being stupid? Other than that (and the much
larger series resistor) the circuit is the same.

I also found on piclist.com:

"Another way to protect against over-voltage is to put a regular diode with
the cathode on the signal line and the anode on the power supply line. Keep
in mind that this will cause the power supply to receive the over-voltage
and may result in damage to other components on the board."

Standard diodes are cheaper... any reason why I can't put ONE Zener in the
power supply and use regular diodes on the 8 pins?

x8:
                      +5
                       |
 +5                    /
_|_                    \
_\ /_                   /
 |    220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>

x1:

 +5
_|_/
//_\ 5.1V
 |
GND

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2003\10\24@174426 by Josh Koffman

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Hi James. I don't have an actual example for you, but in "The Robot
Builder's Bonanza" by Gord McComb, he had a parallel port buffer for the
hobbiest so you wouldn't fry your computer. My copy is unfortunately in
another city, so I can't scan it for you. You could try your local
library though.

Josh
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"James Newton, webhost" wrote:
<big snip>

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2003\10\24@192313 by Bob Blick

face
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Hi James,

My own experience with PC parallel ports has been that they are relatively
tough, and you can communicate to and from a PIC pretty much one-to-one.

The real damage comes from ground voltage differences. If your PC is not
grounded (like if the third prong is missing from the equation somewhere)
then the PC will be 60 volts AC away from ground, and that is where there
can be some issues. People hooking their PC into their TV entertainment
system do sometimes blow up their sound cards.

However, since people get away with hooking printers and PIC programmers
up to their PCs, day in and day out, with hardly ever a problem, I'd
suggest doing the least possible. Like just hooking them together. Or
using a series resistor, and optional zener + diode.

How much this adds to the cost of your device should be a factor.

Of course, there is EMI compliance. Do you need to look at that? Because
filters and protection can be combined.

There are various standard-setting agencies each with multiple levels of
test limits and you could build to suit the one you choose.

Asking if something is protected enough is hard, because, like doctors and
lawyers, the "safe" answer is "not 100 percent safe".

Cheerful regards,

Bob

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2003\10\24@204202 by James Newton, webhost

face picon face
source= http://www.piclist.com/piclist/2003/10/24/192313a.txt?


Bob Blick says: "The real damage comes from ground voltage differences. If
your PC is not grounded (like if the third prong is missing from the
equation somewhere) then the PC will be 60 volts AC away from ground, and
that is where there can be some issues."

Any suggestions on protecting against that sort of thing? Fuse the ground
connection between the PC and device?

"...Of course, there is EMI compliance. Do you need to look at that?..."

No, thank goodness.

Just looking (as you suggest) to do the minimum and hope to stop 90% of the
standard junk that happens.

Thanks for the feedback... I'd actually just really like to know why I have
the series diode going one way and Tomi has it going the other way...
http://www.piclist.com/piclist/2003/10/24/174008a

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2003\10\25@054054 by Rob Hamerling

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Hi James,

James Newton, webhost wrote:

> So, if I want to put out something that people can setup from a PC, a
> temporary connection via the parallel port is probably best. Now the
> question is how do I keep people from bread boarding something that connects
> via my device that will blow up the PC (as well as me probably) when they
> screw up?
>
> I went on a hunt for parallel port isolation and, not being the most
> brilliant analog electronics engineer (ok, so I'm a complete moron in that
> area) I really don't seem to understand most of what I found.

I do control some modelrailroad devices via the parallel port of my PC,
and use optocouplers to protect my PC. See for schematics:
http://www.robh.nl/art/s88lptw.gif. It might be a beginning.

Regards, Rob.

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2003\10\25@090449 by Dennis Crawley

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James, how about optocoupling the port?.

I think It can be done with:
1.- an extra cable from the PC to assure 5v from supply .
2.- buffers like 7404, some resistors (cable balance).
3.- 6n317 opto, VERY fast!
4.- A power supply on the project side.
5.- and I think an extra logic to deal with bi-directionality it will be
needed.

With this structure not only the port is protected, but also the ground
plane is isolated.
I did something similar for RS232 And it works ok! connecting a PIC with a
PC with out Max232.

Dennis.


----- Original Message -----
From: "James Newton, webhost"
Sent: Friday, October 24, 2003 9:38 PM
Subject: Re: [PIC:] safely driving a pic input from a PC Parallel port.


>
> Any suggestions on protecting against that sort of thing? Fuse the ground
> connection between the PC and device?

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2003\10\25@121643 by Bob Axtell

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James, it is NOT that hard to interface safely to PC's.

Data TO THE PC: The PC expects either OPEN COLLECTOR inputs or tri-state
drivers of the Schotty LS (74LSXX) variety, which is extremely durable. The
PIC MAY drive it properly, but is not as durable. I recommend a TTL driver
chip.

Data FROM The PC: The PC sends signals of 0.6V max (low) and 5.0V max
(high) but in some cases the HIGH output might be as low as 3.5V. So it
needs a TTL interface as well.

Overall, I suggest a I/O chip like the 74LS245 which can handle both
directions of the databus. If the databus is NOT being used
bidirectionally, the 74LS244 can act as an interface chip; all input
signals of the LS244 are Schmitt-triggered. Small current limiting
resistors, such as 33ohm, between signals of each systems further increases
overall reliability.

--

To handle NT/XP/Win2K issues, I use Winsoft.Sk IO drivers (C++/Delphi);
they have a very good one for parallel ports that make interfacing Win32
I/O like the good 'ole days. http://www.winsoft.sk . Erik is an exceptional
programmer and makes a product that REALLY works; I've used IOPORT and
COMPORT sucessfully for three years now.

--Bob








At 02:37 PM 10/24/2003, you wrote:
{Quote hidden}

--------------
Bob Axtell
PIC Hardware & Firmware Dev
Tucson, AZ
1-520-219-2363
"I lose some on each sale but make it up in volume."

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2003\10\25@150924 by James Newton, webhost

face picon face
source= http://www.piclist.com/piclist/2003/10/25/054054a.txt?

Rob Hamerling says:I do control some modelrailroad devices via the parallel
port of my PC, and use optocouplers to protect my PC. See for schematics:
http://www.robh.nl/art/s88lptw.gif.

Rob, what optocoupler did you use? What part number I mean.

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2003\10\25@151723 by James Newton, webhost

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source= http://www.piclist.com/piclist/2003/10/25/090449a.txt?

Dennis Crawley says: "3.- 6n317 opto, VERY fast!"

Are you sure of that part number? I can't find it anywhere.

http://www.findchips.com/avail?part=6n317


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2003\10\25@152754 by James Newton, webhost

face picon face
source= http://www.piclist.com/piclist/2003/10/25/121643a.txt?

Bob Axtell says: "Overall, I suggest a I/O chip like the 74LS245 which can
handle both directions of the databus. If the databus is NOT being used
bidirectionally, the 74LS244 can act as an interface chip; all input signals
of the LS244 are Schmitt-triggered. Small current limiting resistors, such
as 33ohm, between signals of each systems further increases overall
reliability."

Based on the pricing, that might just be the best thing... 48 cents in some
packages quantity one from digikey.

But I don't think that would protect against the "AC between grounds"
problem that was mentioned.

Optoisolators sound best, but so far the prices are killer. The 244 may just
be the "sweet spot."

But, I STILL don't understand why my diode ended up in the opposite
direction...

Tomi:
{Quote hidden}

Me:
{Quote hidden}

Huh? huh!? Why? WHY!? WHY AM I BACKWARDS?????

Err... maybe I shouldn't put it exactly that way...

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2003\10\25@153008 by Ishaan Dalal
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Some optocouplers I have seen are quirky in that they have open collector
outputs, which may make things tough if you are going to be sourcing any
current; they also often use bipolars at the output, giving the usual
source/sink asymmetries. I prefer solid-state optical relays instead, that
are "true" switches just like standard relays, and can handle ~50-250 mA in
any direction. Check out the excellent HSR312/HSR412 single SSRs from
Fairchild semi, I've been very happy with them. NAIS/Aromat also make dual
SSRs (8-pin packages) which I have successfully used too. Check them out at
http://www.aromat.com/photomos.htm ; I'm not sure, but they might make
triple or quad relays in 14-pin pkgs too.

The NAIS relays, I remember, had rise/fall times of <~1 uS or so. If you are
looking for mad fast comms, check out the 10-mbps optocouplers from Agilent
or Fairchild - it's xx-2631, a look through the list on Fairchild Semi's
opto page should give you the xx - these have rise/fall times of <~100 ns.

Cheers,
Ishaan

{Original Message removed}

2003\10\25@153624 by Ishaan Dalal

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face
I think he meant the 6N137 (aka HCPL-2630/2631), 10 mbps optocouplers. Note
that these have open-collector outputs.

Cheers,
Ishaan

{Original Message removed}

2003\10\25@154507 by Ishaan Dalal

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From: "James Newton, webhost" <@spam@jamesnewtonKILLspamspamPICLIST.COM>

> Based on the pricing, that might just be the best thing... 48 cents in
some
> packages quantity one from digikey.

Well, any of the fancy SSRs I mentioned is going to set you back at least
$1.25 or so per SSR in small quantities (i.e., a 4-pack would be ~$5).

You might want to try the good old 4N29 oc's (darlington outputs), 6-pin
DIP, $0.14 small qtys from Arrow.

Or, I just found the Fairchild H11A617As, 4-pin DIP, single phototransistor
outputs, $0.16 small qtys from Arrow.

Cheers,
Ishaan

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2003\10\25@160154 by Rob Hamerling

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James Newton, webhost wrote:
> source= http://www.piclist.com/piclist/2003/10/25/054054a.txt?
>
> Rob Hamerling says:I do control some modelrailroad devices via the parallel
> port of my PC, and use optocouplers to protect my PC. See for schematics:
> http://www.robh.nl/art/s88lptw.gif.
>
> Rob, what optocoupler did you use? What part number I mean.

James,
Basically the PC817, but since I needed 4 of these, I took a PC847.
In my case it isn't very critical, since data transfer is not
particularly high speed (approx. 20 KHz).
You can find more info (software) by browsing to
http://www.robh.nl/mklsoft.htm#S88LPT

Regards, Rob.


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2003\10\25@160404 by Bob Blick

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On 25 Oct 2003 at 12:27, James Newton, webhost wrote:
> Optoisolators sound best, but so far the prices are killer. The 244 may just
> be the "sweet spot."

Hi James,

The only problem with the '244 or any other interface chip is that it
doesn't protect your PC parallel port during the plugging and
unplugging process. Only the optoisolators will protect you then.

On the other hand, as long as the shell of your connector is wired
through to ground, you are pretty safe against ground differences
because the shell contacts first and breaks last.

Which brings me back to the series resistor by itself, or a series resistor
followed by a zener to ground and diode to supply if you want to get
fancy.

Oh, another thing. PICs are pretty resistant to latchup, more so than
many logic families you might be tempted to use for an interface.
Although you are safe with LS, HC is highly prone to latchup.

Cheerful regards,

Bob

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2003\10\25@172035 by William Chops Westfield

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> On 25 Oct 2003 at 12:27, James Newton, webhost wrote:
>> Optoisolators sound best, but so far the prices are killer. The 244
>> may just be the "sweet spot."

I'm beginning to see why lots of people don't like using the parallel
port.  The "PC compatible" parallel port is something of a crock, and
propagating that crock into big multi-io chips in irreplaceable spots
on expensive motherboards was silly. :-(

Serial IO at least has a bit of built-in protection in the form
of the drivers (those are still irreplaceable, but more robust.)

USB has the big advantage that it was designed well after the problems
of putting connectable electronics in the hands of consumers were
better understood.  USB is wonderful stuff, I think.

Add the fact that parallel ports are also disappearing from modern PCS,
and the difficulty in accessing the ports from user programs in
modern operating systems, and "anything else" starts to look pretty
attractive as your protection circuitry costs start to go up.

(or - the easiest way to protect a PC's parallel port is to put it
at the end of a USB cable.  Those run about $25.  Does anyone happen
to know of (or whether) any of these are usable in the "flexible"
fashion of a normal parallel port (being able to set/read individual
signals, etc.)  Documented?)

BillW

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2003\10\27@055700 by Alan B. Pearce

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>Dennis Crawley says: "3.- 6n317 opto, VERY fast!"

I think he means the 6N137, which is a Darlington transistor output.

However I prefer the HCPL2300 which has an internal opto receiver, and comes
out as an open collector digital signal, with a 1k pull up resistor
available for connection. Requires 5V supply for the receiver side, and is a
heap faster than the 6N137. Both devices are Agilent ones.

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2003\10\27@085445 by Jim Monteith

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I talk to PICs daily through the parallel port on windows 2000.  For a
manufacturing process, I created a program that writes the code to a PIC,
and verifies it, also read and write from/to EEprom.  The two keys here are
using a drive program like OpenPort which gives you direct access to the
parallel port, and then how you write to the port.  You have to write to the
port address which is usually 0x0378.  Beyond this, it depends on what you
want to do.  You really need to study the pinout of the parallel port, and
decide which pins you want to use.

It's not that hard to do, and it is a very stable way to talk to the PIC.


Jim

{Original Message removed}

2003\10\27@100641 by George Sacco

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Jim, where can I get the program OpenPort

Thanks
george

-----Original Message-----
From: pic microcontroller discussion list
[RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU] On Behalf Of Jim Monteith
Sent: Monday, October 27, 2003 8:52 AM
To: spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
Subject: Re: [PIC:] safely driving a pic input from a PC Parallel port.

I talk to PICs daily through the parallel port on windows 2000.  For a
manufacturing process, I created a program that writes the code to a
PIC,
and verifies it, also read and write from/to EEprom.  The two keys here
are
using a drive program like OpenPort which gives you direct access to the
parallel port, and then how you write to the port.  You have to write to
the
port address which is usually 0x0378.  Beyond this, it depends on what
you
want to do.  You really need to study the pinout of the parallel port,
and
decide which pins you want to use.

It's not that hard to do, and it is a very stable way to talk to the
PIC.


Jim

-----Original Message-----
From: pic microcontroller discussion list
[TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU]On Behalf Of James Newton, webhost
Sent: Friday, October 24, 2003 16:37
To: RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
Subject: [PIC:] safely driving a pic input from a PC Parallel port.


I've been thinking about how things get connected between PC's and PIC's
lately and overall what I can see is:
USB: Nice! But not the lowest possible cost and not supported on some
old
PC's Also, requires some driver development on the PC side... not hard,
but
still.

RS232: Nice... But you can run out of ports, some current laptops don't
even
have serial ports (!) and you have to do all the darn configuration on
the
PC to make sure all the settings are right. Has anyone been able to get
a
USB to RS232 adapter to work right? I can never get the PC application
to
see the port.

Parallel: Sigh... NT and up driver problems (IF you don't simulate a
printer
with the PIC) but there are solutions for that. One the one port usually
and
I've seen laptops without parallel ports. USB to parallel adapters are
really shaky... has anyone used one to talk to anything other than a
printer? The two I've tried don't show up as LPT ports on the PC and so
old
applications don't see them.

Of all the above, probably the most reliable, simple, and cheapest for
us
little guys is the parallel port (this is IMHO and I'm sure the point
can be
argued 20 ways from Sunday).

So, if I want to put out something that people can setup from a PC, a
temporary connection via the parallel port is probably best. Now the
question is how do I keep people from bread boarding something that
connects
via my device that will blow up the PC (as well as me probably) when
they
screw up?

I went on a hunt for parallel port isolation and, not being the most
brilliant analog electronics engineer (ok, so I'm a complete moron in
that
area) I really don't seem to understand most of what I found.

I understand that the port can sink 24mA and only source 2.6mA so
obviously
its a good idea to drive your loads from the port, through the load,
then to
+5v rather than ground.

Safe current limiting can be had with a series resistor of 1.6k or more
right? (4.2V / 2.6mA) But that doesn't leave much drive... so you can
add a
driver, but wouldn't it be better to prevent the pin from sourcing
anything
by putting a diode long with the resistor in series and then reducing
the
resistor to 175 or more ohms (4.2V / 24mA)?


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                       /
      220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>
_|_/
//_\ 5.1V
 |
GND

A pull-up resistor is added to the output to drive TTL or other loads
and a
Zener could be added to ground to prevent over voltage or driving the
pin in
reverse. That seems to me to be a nice little, very low cost, easy to
work
with, circuit to provide some basic protection for kids or students to
tack
things onto.

Not wanting to assume I have a clue, I searched the internet for other
solutions. Tomi Engdahl says:

"The following circuit uses two 1N4148 diodes to protect parallel port
against higher than +5V signals and also against wrong polarity signals
(power on the circuit is accidentally at wrong polarity. "

         Diode
         1N4148  4.7K
parallel  >-|>|-+--\/\/\/--etc...
port data       |
pin             +-|<|-+
           1N4148    |
parallel  >-----------+
port ground           |
                   Ground

"Adding even more safety idea: Replace the 1N4148 diode connected to
ground
with 5.1V Zener diode. That diode will then protect against over-voltage
spikes and negative voltage at the same time. "

Ok, he has his series diode in the exact opposite direction. Could
somebody
please tell me if I'm just being stupid? Other than that (and the much
larger series resistor) the circuit is the same.

I also found on piclist.com:

"Another way to protect against over-voltage is to put a regular diode
with
the cathode on the signal line and the anode on the power supply line.
Keep
in mind that this will cause the power supply to receive the
over-voltage
and may result in damage to other components on the board."

Standard diodes are cheaper... any reason why I can't put ONE Zener in
the
power supply and use regular diodes on the 8 pins?

x8:
                      +5
                       |
 +5                    /
_|_                    \
_\ /_                   /
 |    220R    1N4148   \
>-o---,/\/\/'----|<-----o-----=>

x1:

 +5
_|_/
//_\ 5.1V
 |
GND

---
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2003\10\27@110827 by Jim Monteith

flavicon
face
Hi James,

Below are 4 sources for different "port openers", and then just a pretty
good place to start reading about parallel port programming.  I searched for
"program windows 2000 parallel port" on google and came up with lots of
hits.  I was wrong with the name I told you.  I am using "userport".  It
works pretty well.  You have to be careful with your timing because what you
are doing is passing off your data to the userport driver which then sends
it to the port.  I have noticed that sometimes I lose data because I'm
trying to read or write too fast.  Just be aware of that.

Hope this helps,

Jim

http://www.zeecube.com/IOAccess/index.htm

http://www.beyondlogic.org/porttalk/porttalk.htm

www.embeddedtronics.com/public/Electronics/minidaq/userport/UserPort.
zip

http://www.logix4u.net/inpout32.htm

http://www.lvr.com/parport.htm

{Original Message removed}

2003\10\27@121728 by Dennis Crawley

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face
Alan, Yes, I mean that.
I've planned to do something similar to James but to serial port oriented.




Regards,
[OTps:] I can't figured out what lost us that match (Ireland16-Argentina15).
I wish I had been at pub just to see the Irish party.


From: "Alan B. Pearce"
To: <RemoveMEPICLISTspam_OUTspamKILLspamMITVMA.MIT.EDU>
Sent: Monday, October 27, 2003 7:55 AM
Subject: Re: [PIC:] safely driving a pic input from a PC Parallel port.


> >Dennis Crawley says: "3.- 6n317 opto, VERY fast!"
>
> I think he means the 6N137, which is a Darlington transistor output.
>
> However I prefer the HCPL2300 which has an internal opto receiver, and
comes
> out as an open collector digital signal, with a 1k pull up resistor
> available for connection. Requires 5V supply for the receiver side, and is
a
> heap faster than the 6N137. Both devices are Agilent ones.
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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