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'[EE]: Real newbie question'
2001\12\28@084446 by Keith Christopher

picon face
Question for the list, when setting up a circuit, how
does one know when to use a capacitor _vs_ a resistor
?






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2001\12\28@090559 by Sean Alcorn - Avion Sydney

flavicon
face
on 29/12/01 12:42 AM, Keith Christopher at spam_OUTkeithc_21211TakeThisOuTspamYAHOO.COM wrote:

Hi Keith,

> Question for the list, when setting up a circuit, how
> does one know when to use a capacitor _vs_ a resistor

I'm not quite as new as you, but pretty new.

Have you had any background in electronics? It depends what you want to do.

With Micros, resistors are generally used as "pull-ups" or "pull-downs" -
ie; you use say a 4K7 or a 10K resistor to "pull-up" an input to the
positive (VCC) rail, then you can use a switch or other device to "pull" the
input to ground. You can also do it in reverse - use the resistor to pull
the input low (to the ground rail), then have the switch or othe input
device pull the input high.

Resistors are also used for current limiting (ie; driving LEDs, transistors
etc) and a capacitor/resistor combination can also be used as the cheapest
form of external clock.

Capacitors are used to filter your power supply (large electrolytics) and
smaller ceramic caps can be used to filter noise from your inputs and are
necessary to be used in conjunction with external crystals, reset circuits
etc.

My best advice I can give you is to buy some books - projects books for the
PIC would be preferred - and study the schematics and read why certain
components were chosen and/or not used and apply these rules to your own
circuits.

Hope I have been of some help.

Regards,

Sean

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2001\12\28@092127 by uter van ooijen & floortje hanneman

picon face
> Question for the list, when setting up a circuit, how
> does one know when to use a capacitor _vs_ a resistor

When you set up a circuit you have a particular goal in mind. Without
mentioning the goal you cannot choose between an electrical circuit and a
chemical compound, or even a prayer, let alone between a cap and an R. So:
what do you want with your circuit?

Wouter van Ooijen

Van Ooijen Technische Informatica: http://www.voti.nl
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2001\12\28@104852 by Thomas C. Sefranek

face picon face
What circuit?

You use capacitors to filter, decouple, integrate, differentiate and
other stuff.
You use resistors to drop voltage, limit current, make heat and other stuff.
The answer I provide is as obscure as your question,

but it is an attempt at a serious answer.

Keith Christopher wrote:

{Quote hidden}

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2001\12\28@133542 by Mina Nichols

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face
When driving a car, how does one know when to use a left turn _vs_ a right?
-Mina

-----Original Message-----
From: pic microcontroller discussion list
[PICLISTspamKILLspamMITVMA.MIT.EDU]On Behalf Of wouter van ooijen & floortje
hanneman
Sent: Friday, December 28, 2001 1:07 AM
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject: Re: [EE]: Real newbie question


> Question for the list, when setting up a circuit, how
> does one know when to use a capacitor _vs_ a resistor

When you set up a circuit you have a particular goal in mind. Without
mentioning the goal you cannot choose between an electrical circuit and a
chemical compound, or even a prayer, let alone between a cap and an R. So:
what do you want with your circuit?

Wouter van Ooijen

Van Ooijen Technische Informatica: http://www.voti.nl
Jal compiler for PIC uC's:  http://www.voti.nl/jal
PICs kopen? http://www.voti.nl/shop

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2001\12\28@144421 by Jinx

face picon face
> How does one know when to use a capacitor _vs_ a resistor ?

Hmmm, tricky one. Depends whether you want to offend your
ears or your nose. Caps go BANG, resistors make a lot of
smelly smoke. Personally I'd go for a triac - can take an eye
out, bust an eardrum and they stink, all for 50c. Now that's
value for money. Or two bangs for a buck if you please

You'll have already seen the posts asking fo' mo' info'

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2001\12\28@153341 by Keith Christopher

picon face
I guess the question is:

I would use a capacitor when (or if) . . .

I would use a resistor when (or if ) . . .



--- "Thomas C. Sefranek" <EraseMEtcsspam_OUTspamTakeThisOuTCMCORP.COM> wrote:
{Quote hidden}

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2001\12\28@160629 by Aaron Blohowiak

picon face
checkit:

faculty.millikin.edu/~jaskill.nsm.faculty.mu/newphyslab9.html
www.deas.harvard.edu/courses/es154/pages/nicetut/
www.pcguide.com/intro/fun/components-c.html
www.gae.ucm.es/~padilla/extrawork/electronics.html
http://www.saburchill.com/tech/adobe/001.pdf

(if i can find all this in 5 min using google, so can you =) [assuming you
have a cable modem, maybe 10 mins if you dont;)])

Aaron
[snip]
> I guess the question is:
>
> I would use a capacitor when (or if) . . .
>
> I would use a resistor when (or if ) . . .
[/snip]

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2001\12\28@162427 by Jinx

face picon face
> I guess the question is:
>
> I would use a capacitor when (or if) . . .
>
> I would use a resistor when (or if ) . . .

You can't really make that generalisation. The examples
below may depend on whether you are working with AC or DC

Resistors limit current and drop voltage in AC - but so can
capacitors

Filters / integrators and other frequency-dependent functions
will/can be a combination of R and C

Both R and C can be used as voltage dividers

*** Present a practical problem to get specific advice ***

> > You use capacitors to filter, decouple, integrate,
> > differentiate and other stuff.

> > You use resistors to drop voltage, limit current,
> > make heat and other stuff.

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2001\12\28@172143 by Thomas C. Sefranek
face picon face
Keith Christopher wrote:

> I guess the question is:
>
> I would use a capacitor when (or if) . . .



... if I want :
to filter, decouple, integrate,
> differentiate and
> other stuff.

>
>
> I would use a resistor when (or if ) . . .

I want...
to drop voltage, limit current,
{Quote hidden}

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2001\12\28@222837 by Keith Christopher

picon face
Everyone,

  I appreciate your input 1st and foremost. I am new
to designing circuits, and was looking to see if there
is a general rule of thumb. What I am looking to do is
get input from one serial (232) input and using an AND
gate switch between two outputs using a PIC to set the
second input of the gate high (the 232 input on the
other.) I know I cannot simply wire them straight
together, I will need some resistance and capacitance
somewhere, I was trying to figure out where without
having someone else answer the question for me.

This is why I was looking for a general rule of thumb
on using resistors and caps.

Also LED get REALLY REALLY bright before they blow.
8-)

Keith


--- Aaron Blohowiak <spamBeGoneellipsisspamBeGonespamHOME.COM> wrote:
> checkit:
>
>
http://faculty.millikin.edu/~jaskill.nsm.faculty.mu/newphyslab9.html
>
www.deas.harvard.edu/courses/es154/pages/nicetut/
> http://www.pcguide.com/intro/fun/components-c.html
>
www.gae.ucm.es/~padilla/extrawork/electronics.html
{Quote hidden}

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2001\12\28@232606 by Jinx

face picon face
> is a general rule of thumb. What I am looking to do is
> get input from one serial (232) input and using an AND
> gate switch between two outputs using a PIC to set the
> second input of the gate high (the 232 input on the
> other.) I know I cannot simply wire them straight
> together

If I get the gist of what you are saying, and I'm not terribly
sure I do, then more than likely capacitors are out. More
than like so are resistors. It sounds like the sort of circuit
that could be wired-OR, using signal diodes. It would be
very helpful if you could give a lot more detail, even down
to chips and pin numbers, so that the list can get a handle
on what it is you want to do. We've all asked bunny questions,
don't worry about it

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2001\12\29@004620 by Dale Botkin

flavicon
face
On Fri, 28 Dec 2001, Keith Christopher wrote:

>    I appreciate your input 1st and foremost. I am new
> to designing circuits, and was looking to see if there
> is a general rule of thumb. What I am looking to do is
> get input from one serial (232) input and using an AND
> gate switch between two outputs using a PIC to set the
> second input of the gate high (the 232 input on the
> other.) I know I cannot simply wire them straight
> together, I will need some resistance and capacitance
> somewhere, I was trying to figure out where without
> having someone else answer the question for me.

Well, if you convert the RS232 levels to TTL/CMOS with a line receiver
first, yes you can connect things directly without resistors or
capacitors.  Hence my love of digital circuits!  8-)

> Also LED get REALLY REALLY bright before they blow.
> 8-)

Ah, so you have discovered Noise Emitting Diodes?  Just like an LED, but
used to indicate a high current condition.  Emits a loud SNAP sound
(once).

Dale

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2001\12\29@011432 by Keith Christopher

picon face
What I'm doing is having one PC on input using a
Max232 and want the ability to switch between 2 output
ports. i.e. PC to comp#1 of if I type a string ~1 the
pic switches the output to comp#2.

232
| |
| ---PIC---
| |  |     |
| AND 1    |
|          |
AND 2------

There are 4 AND gates which go to tx and rx on comp 1
and comp2


Keith

--- Jinx <TakeThisOuTjoecolquittEraseMEspamspam_OUTCLEAR.NET.NZ> wrote:
{Quote hidden}

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2001\12\29@025723 by Aaron Blohowiak

picon face
you guys and gals ARE wearing eye protection, right? (thankfully i was
wearing mine tonight.. while soldering a little thing fizzled up and hit my
saftey glasses. I went to dremel the perf board, and a CLOUD of dust came
out where i was cutting.. woulda really messed me up.) Anyway, its not that
big of a pain, and how much are your eyes worth to you?

Its a good idea,
Aaron

> > Also LED get REALLY REALLY bright before they blow.
> > 8-)
>
> Ah, so you have discovered Noise Emitting Diodes?  Just like an LED, but
> used to indicate a high current condition.  Emits a loud SNAP sound
> (once).

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2001\12\29@031040 by Kathy Quinlan

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face
As an Au Occupation  health and safety rep for my company, I insist that all
employees (all one of them) wear normal glasses and safety glasses.......


Seriously though from personal experience, being blind is no fun, (medical
problem had the side effect of tempary blindness for a week, the novelty
wore off after a day :o(   )

it amazes me the number of engineers who wear glasses and think they are
protected, but these are not enough, as good safety glasses stop bits
getting around the lenes, one place I worked, we all had green tinted safety
glasses, it did not affect colour recognition, but apparently improved
visibility in low light areas ???

Regards,

Kat.

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{Original Message removed}

2001\12\29@050606 by uter van ooijen & floortje hanneman

picon face
>    I appreciate your input 1st and foremost. I am new
> to designing circuits, and was looking to see if there
> is a general rule of thumb. What I am looking to do is
> get input from one serial (232) input and using an AND
> gate switch between two outputs using a PIC to set the
> second input of the gate high (the 232 input on the
> other.) I know I cannot simply wire them straight
> together, I will need some resistance and capacitance
> somewhere, I was trying to figure out where without
> having someone else answer the question for me.

I don't realy follow what you want to do. Try do describe what you want to
do (switch between two RS232 inputs?) not how you intend to do it (using an
AND gate). That way you might get other solutions, which can be remarkably
simpler that your (or my) first thoughts.

Wouter van Ooijen

Van Ooijen Technische Informatica: http://www.voti.nl
Jal compiler for PIC uC's:  http://www.voti.nl/jal
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2001\12\29@152811 by uter van ooijen & floortje hanneman

picon face
> What I'm doing is having one PC on input using a
> Max232 and want the ability to switch between 2 output
> ports. i.e. PC to comp#1 of if I type a string ~1 the
> pic switches the output to comp#2.

That is a good attempt at 'specificaion' instead of 'solution description'.
But depepending on what you really want there are still a lot of solutions!

1. You did not mention the boundary conditions, but assuming that you want
to send to two devices from a PC, and the devices do not need the handshake
signals, you could use one of those to control a relais. With a small relais
you might be able to drive it directly from the RS232 handshake signals.
Otherwise a T + R + power supply will accomodate almost any relais. But
don't forget the diode! (That is neither a C nor an R, but still a very
usefull component, sepecially in this case!)

2. Assuming that you can not use a handshake signal the easiest route would
be to convert RS232 to TTL, feed that to the PIC, and let the PIC control a
relais. No need to convert TTL back to RS232, a 1488 (or is it 1489?) is
much cheaper than a max232.

3. Assuming that you are allergic to relais you could input the TTL level
serial input to a PIC, and let the PIC copy the signal to the selected
output. No nand or other gates required.

4. Assuming that you are very fond of gates, the PIC can of course be kept
stupid, and just provide a signal to the gates.

Wouter van Ooijen

Van Ooijen Technische Informatica: http://www.voti.nl
Jal compiler for PIC uC's:  http://www.voti.nl/jal
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2001\12\30@012430 by Alexandre Domingos F. Souza

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face
>Hmmm, tricky one. Depends whether you want to offend your
>ears or your nose. Caps go BANG, resistors make a lot of
>smelly smoke. Personally I'd go for a triac - can take an eye
>out, bust an eardrum and they stink, all for 50c. Now that's
>value for money. Or two bangs for a buck if you please

       Nah, old selenium rectifiers. They BANG better, smell worse and has a nice blue color, like the SGI workstations. These are my prefered burns in the "crazy lab", along with some electrolytic capacitors and zener diodes.


---8<---Corte aqui---8<----

Alexandre Souza
RemoveMEtaitospamTakeThisOuTterra.com.br
http://planeta.terra.com.br/lazer/pinball/

---8<---Corte aqui---8<----

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2001\12\30@012439 by Alexandre Domingos F. Souza

flavicon
face
>You can't really make that generalisation. The examples
>below may depend on whether you are working with AC or DC
>Resistors limit current and drop voltage in AC - but so can
>capacitors
>Filters / integrators and other frequency-dependent functions
>will/can be a combination of R and C
>Both R and C can be used as voltage dividers
>*** Present a practical problem to get specific advice ***
>> > You use capacitors to filter, decouple, integrate,
>> > differentiate and other stuff.
>> > You use resistors to drop voltage, limit current,
>> > make heat and other stuff.

       Why not just tell the new-b to go to http://www.epanorama.net and search for good "electronics 101" texts? :o)))


---8<---Corte aqui---8<----

Alexandre Souza
taitoEraseMEspam.....terra.com.br
http://planeta.terra.com.br/lazer/pinball/

---8<---Corte aqui---8<----

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2001\12\30@013125 by Alexandre Domingos F. Souza

flavicon
face
>   I appreciate your input 1st and foremost. I am new
>to designing circuits, and was looking to see if there
>is a general rule of thumb. What I am looking to do is
>get input from one serial (232) input and using an AND
>gate switch between two outputs using a PIC to set the
>second input of the gate high (the 232 input on the
>other.) I know I cannot simply wire them straight
>together, I will need some resistance and capacitance
>somewhere, I was trying to figure out where without
>having someone else answer the question for me.

       Simple as it can be ;o)

       We stopped using the AND gates as "controlled switches". Done that, you need to MIX or to SELECT the output? If you just want to mix the output (since only one will be connected at any given time), you just put an OR gate after the two AND gates ;o)

       Don't forget to connect unused inputs to ground!


---8<---Corte aqui---8<----

Alexandre Souza
EraseMEtaitospamterra.com.br
http://planeta.terra.com.br/lazer/pinball/

---8<---Corte aqui---8<----

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2001\12\30@021037 by Alexandre Domingos F. Souza

flavicon
face
       =(^.^)=,

>it amazes me the number of engineers who wear glasses and think they are
>protected, but these are not enough, as good safety glasses stop bits
>getting around the lenes, one place I worked, we all had green tinted
>safety
>glasses, it did not affect colour recognition, but apparently improved
>visibility in low light areas ???

       I always use then. These are C H E A P (around Us$ 2 here) and has protections around the lenses - it looks like a diver mask - very confortable (eh...not too much) and doesn't affect color rendition and like. I always use it when power grinding, using a dremel or like. :o)

       I'm a good boy, hehehe ;oD


---8<---Corte aqui---8<----

Alexandre Souza
RemoveMEtaitoEraseMEspamEraseMEterra.com.br
http://planeta.terra.com.br/lazer/pinball/

---8<---Corte aqui---8<----

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2001\12\30@021058 by Alexandre Domingos F. Souza

flavicon
face
>1. You did not mention the boundary conditions, but assuming that you want
>to send to two devices from a PC, and the devices do not need the handshake
>signals, you could use one of those to control a relais. With a small
>relais
>you might be able to drive it directly from the RS232 handshake signals.
>Otherwise a T + R + power supply will accomodate almost any relais. But
>don't forget the diode! (That is neither a C nor an R, but still a very
>usefull component, sepecially in this case!)

       Been there, done that. It's easier to use two AND gates, and switch its outputs! I'll scan and post a NMEA signal selector I did some years before, that is something like he wants to do, and everyone will understand




---8<---Corte aqui---8<----

Alexandre Souza
RemoveMEtaitospam_OUTspamKILLspamterra.com.br
http://planeta.terra.com.br/lazer/pinball/

---8<---Corte aqui---8<----

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2001\12\30@035650 by Jinx

face picon face
> getting around the lenes, one place I worked, we all had green
> tinted safety glasses, it did not affect colour recognition, but
> apparently improved visibility in low light areas ???
>
> Regards,
>
> Kat.

Tried walking around the house tonight with my welding goggles.
Shins currently in transit to you for repair and return forthwith

(but seriously, human eyes respond best to green, note that
night-vision screens are green, there could be something in
what you say. btw, get the lead out with those shins, my feet
look stoopid sitting in a pair of shoes on their own and I can't
reach light switches)

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2001\12\30@051047 by Kathy Quinlan

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----- Original Message -----
From: "Jinx" <RemoveMEjoecolquittTakeThisOuTspamspamCLEAR.NET.NZ>
> Tried walking around the house tonight with my welding goggles.
> Shins currently in transit to you for repair and return forthwith
>
> (but seriously, human eyes respond best to green, note that
> night-vision screens are green, there could be something in
> what you say. btw, get the lead out with those shins, my feet
> look stoopid sitting in a pair of shoes on their own and I can't
> reach light switches)
>
Giggle that will teach ya ;o) these were not as dark as welding goggles,
more a very light almost clear tint :o)


/me thinks we could replace your legs with titanium ones, with pic controled
servo's to simulate the knee and ankle joint ;o)

Hey was watching the rally coverage today, and they had the Targa <sp?> 2000
on, got to see some nice NZ country side :o)


Regards,

Kat.



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2001\12\30@152945 by Keith Christopher

picon face
I want to be able to select, this is where the PIC
comes in. I want the PIC to listen and if it sees a
special key sequence it will turn off one of the tx/rx
AND gates and turn on the other tx/rx AND gates.
Allowing communication to the other computer.

Keith

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2001\12\30@154522 by Jinx

face picon face
> I want to be able to select, this is where the PIC
> comes in. I want the PIC to listen and if it sees a
> special key sequence it will turn off one of the tx/rx
> AND gates and turn on the other tx/rx AND gates.
> Allowing communication to the other computer.

A possible problem is the signal voltage. If your PCs
are using +/- 12V levels (ie not 5V) then you won't be
able to pass the comms through a standard gate. If
you can get a +/- power supply from the MAX chips it's
possible you could use 4066's or 4052's to select and
route the signals

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2001\12\30@160910 by Keith Christopher

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but the 232 operates in the 5v range, what if I put 3
232's one on each computer and use the AND gate. this
should keep the traffic @ 5v level correct ?



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2001\12\30@161528 by Jinx

face picon face
> > > comes in. I want the PIC to listen and if it sees
> > > a special key sequence it will turn off one of the

Where is the special key sequence coming from ? From
the PC or typed into the PIC ?

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2001\12\30@161542 by Keith Christopher

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OK looking at the data sheet for the max 235, it has
232 ins and TTL/CMOS outputs, if I use this couldn't I
keep my voltage levels right ? It appears as though
it's taking the +- 12v and outputting TTL.


Keith
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2001\12\30@162152 by Keith Christopher

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part 1 693 bytes content-type:text/plain; charset=us-asciiFrom the PC through a MAX 232 here's a rough (I mean
rough) block on the circuit


Keith
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part 2 15544 bytes content-type:image/pjpeg; name="logic circuit.JPG" (decode)


part 3 131 bytes
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2001\12\30@170348 by Jinx

face picon face
part 1 560 bytes content-type:text/plain; (decoded 7bit)

Is this getting close ?

In this circuit, with Ctrl=0 the PIC can listen to PC1, as the
pull-up will enable traffic through the top two gates. If Ctrl=1
then that will disable traffic from PC1 and enable it for PC2

You could use individual enable controls for each PC (if you
expand to more than 2 PCs) if you can assure that all will not
be turned on together and so give data collisions/mixing

My assumption was that you were saving costs by multiplexing
the more expensive MAX chip, hence my query about voltage
levels


part 2 3946 bytes content-type:image/gif; (decode)


part 3 131 bytes
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2001\12\30@170810 by Keith Christopher

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Yes that's SO close. it is missing however the input
PC.

1 pc to connect to 2 different PC's


KEith
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2001\12\30@175924 by Jinx

face picon face
> Yes that's SO close. it is missing however the input
> PC.
>
> 1 pc to connect to 2 different PC's

OK, now we're getting down to it. Both of these answers
have the gates/232 direction flipped, as it's now 1 into 2,
not 2 into 1

Does the source PC decide which other PC it talks to
or does the PIC decide ?

If the source PC decides, possible solution - keep the
circuit pretty much as it is and use one of the handshake
lines of the source PC to do the switching

If the PIC decides then you can use a slight re-arrangement
of the circuit I sent. Substitute the source PC for the PIC, use
the PIC to monitor Tx from the source PC and enable the
appropriate pair gates

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