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'[EE:] Max232 problems'
2004\08\02@215603 by Anthony Toft

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Could someone take a look at

http://cowshed.8m.com/MAX232.html And tell me why it's not working...

I have scoped the lines and the serial port (going into R1in) has a nice
hard edged +-12v trace (I am sending repeated 'A's) the output side,
R1out it reading decidedly 0v.

I have checked for shorts on the boards and there doesn't seem to be
any.

Comments please

Anthony
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2004\08\02@220226 by Bob Axtell

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I think the negative side of C12 should go to VCC, NOT GND.

--Bob

Anthony Toft wrote:
{Quote hidden}

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2004\08\02@222350 by Victor Faria

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I believe c12  should go pin2 and vcc
victor
----- Original Message -----
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Sent: Monday, August 02, 2004 9:55 PM
Subject: [EE:] Max232 problems


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2004\08\02@223630 by David Duffy

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Anthony Toft wrote:

>Could someone take a look at
>
>http://cowshed.8m.com/MAX232.html And tell me why it's not working...
>
>I have scoped the lines and the serial port (going into R1in) has a nice
>hard edged +-12v trace (I am sending repeated 'A's) the output side,
>R1out it reading decidedly 0v.
>
>I have checked for shorts on the boards and there doesn't seem to be
>any.
>

Do you have a common earth between the PC and your circuit?
C12 should really go to Vcc but your way should still work.
Have you checked that incoming RS232 waveform *at* pin 13?
David...

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2004\08\03@001744 by Anthony Toft

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> Do you have a common earth between the PC and your circuit?

Yes, they are grounded together at the input header, and via the power
cords (unintentional)

> C12 should really go to Vcc but your way should still work.

I saw it this way on the Wisp the only difference is that Wouter has his
unused RxOut grounded through a diode which I have left floating

> Have you checked that incoming RS232 waveform *at* pin 13?

Yes, measured at the MAX pin, it's running about +-10v.

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2004\08\03@002215 by David Duffy

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Anthony Toft wrote:

{Quote hidden}

How about the supply to the MAX232 chip? Is it 5V?
Also what voltages are on pins 2 and 6 of the MAX232 ?
David...

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2004\08\03@022113 by Roland

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>
>Yes, they are grounded together at the input header, and via the power
>cords (unintentional)
>

not sure what you mean 'via the power cords' If you're connected to a PC,
the only link should be pin5 on the D9. Careful not to bring in ground
loops, and don't connect 0V to chassis earth.

Change the C12 connection to spec, then
Remove the pic, put the 232 chip in, apply power and do DC checks;
pin 16, 5V
pin 15, 0V, check a correct link to battery 0V
pin 2, at least +8V
pin 6, at least -8V

most importantly;
put meter on pin 14 and conect pin 11 to 0V, then 5V. Pin 14 should swing
+, -  8V
and
put meter on pin 12 and conect pin 13 to 0V, then 5V. Pin 12 should swing
+5V , 0V
if these fail your 232 chip is faulty

The Rxout(pin9) should'nt be grounded, it's an output. The only thing to
ground is pin10, but even for that there are weak internal pull-ups. Leave
pin 8 free.

Regards
Roland Jollivet

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2004\08\03@075449 by Olin Lathrop

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Anthony Toft wrote:
> http://cowshed.8m.com/MAX232.html

You are missing the bypass cap from pin 16 to ground.

> And tell me why it's not working...

Define "not working".

> I have scoped the lines and the serial port (going into R1in) has a
> nice hard edged +-12v trace (I am sending repeated 'A's) the output
> side, R1out it reading decidedly 0v.
>
> I have checked for shorts on the boards and there doesn't seem to be
> any.

Verify that the charge pumps are working.  Pin 2 should be 9-10V, and pin 6
should be -8 to -10 volts.


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2004\08\03@080304 by Olin Lathrop

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> C12 should really go to Vcc

There is nothing wrong with tying it to ground.  I always do it this way.

The charge pump output voltage (which is what's at pin 2) is ultimately used
in a ground-referenced way.  Tying the cap to Vcc puts the impedence of the
Vcc supply in series with the charge pump output.

The only difference is that the cap must now tolerate twice the voltage, but
that's no big deal when it's 10 instead of 5 volts and 100nF.

That brings up another point, what values are the four caps?


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2004\08\03@082209 by Dave Tweed

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Olin Lathrop <EraseMEolin_piclistspamspamspamBeGoneEMBEDINC.COM> wrote:
> > C12 should really go to Vcc
>
> There is nothing wrong with tying it to ground.  I always do it this way.

But it does make a difference. See the first three questions (and their
answers) on this page:
  http://www.circuitcellar.com/library/eq/156/index.htm

The output impedance of the charge pump is approximately halved by the Vcc
connection, and this is (or should be) an order or two of magnitude higher
than the 5V supply impedance, which is negligible in comparison.

-- Dave Tweed

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2004\08\03@082416 by Anthony Toft

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> There is nothing wrong with tying it to ground.  I always do it this way.

The TI MAX232 app notes says to couple it to ground, the note says it
can also go to Vcc.

All the caps are 1uF (the original drawing had the values, but when I
substituted the packages to make the board fit, I forgot to put them
back in)

I went through Roland's check this check that email and went and touched
all the pins with the iron and it seems to be working (it sends
characters to the PIC) However the PIC is having problems that shout of
a mismatched baud rate with garbled characters on the display, although
some come out OK

Thanks for all the assistance guys, this hobby is oodles of fun, and
cooler than word can describe, but I still don't have the experience to
debug it when it doesn't work.

Anthony
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2004\08\03@091824 by Russell McMahon

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> > > C12 should really go to Vcc

> > There is nothing wrong with tying it to ground.  I always do it this
way.

> But it does make a difference. See the first three questions (and their
> answers) on this page:
>    http://www.circuitcellar.com/library/eq/156/index.htm
>
> The output impedance of the charge pump is approximately halved by the Vcc
> connection, and this is (or should be) an order or two of magnitude higher
> than the 5V supply impedance, which is negligible in comparison.

I appreciate that Dave wrote the technical note he refers to. I'm afraid I
have to disagree with the second part of it.

- The part about startup is correct. When returned to Vcc the capacitor
rises to Vcc as supply is applied. Startup is slightly shorter.

- I personally do not believe the second part. Provided that supply
decoupling is good, Vcc and ground are separated by zero impedance. While
this will not be quite so in practice, it should be close enough that there
is a negligible difference between the two situations. The difference is not
able to be analytically determined from the information available and is not
a factor of 2.



       Russell McMahon

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2004\08\03@115217 by Denny Esterline

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> That brings up another point, what values are the four caps?
>
>

As long as we're on (ok, near :-) the subject, what's the upper limit on
the size of the caps?

I've got about 500 leftover 47 uF caps and maybe 50 22 uF caps, are these
tolerable or will they cause problems.

-Denny

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2004\08\03@115835 by Olin Lathrop

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> I've got about 500 leftover 47 uF caps and maybe 50 22 uF caps, are
> these tolerable or will they cause problems.

Different MAX232 variants have different cap specs, but none of them that
large as I recall.  Note that an output short circuit puts a lot more stress
on the charge pump pass elements when large caps are used.  I sortof
remember 1uF is the upper limit.  See data sheet.


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2004\08\03@165614 by Denny Esterline

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This doesn't seem to have made it the first time, apologies if you get it
twice:


> > I've got about 500 leftover 47 uF caps and maybe 50 22 uF caps, are
> > these tolerable or will they cause problems.
>
> Different MAX232 variants have different cap specs, but none of them that
> large as I recall.  Note that an output short circuit puts a lot more
stress
> on the charge pump pass elements when large caps are used.  I sortof
> remember 1uF is the upper limit.  See data sheet.
>

That's the issue, the data sheet doesn't say. To quote the datasheet for
the DS14C232 (national's equivalent)

External capacitor connection pins. Recommended capacitor:
1.0 5F (6.3V). Capacitor value should be larger than
1 5F.

And the best the Maxim Datasheet says is:

the MAX232E and MAX241E require 15F
capacitors, although in all cases capacitors up to 105F
can be used without harm.
Driving Multiple Receivers

But then it goes on to say:

If in doubt, use capacitors with a larger (e.g., 2x) nominal value.

So basically an upper limit is not clearly specified.
-Denny

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2004\08\06@203537 by Rich

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You might want to check the following site:
http://www.maxim-ic.com/appnotes10.cfm/filter/partnumber

You will find a collection of appropriate app notes.  Good Luck.

{Original Message removed}

2004\08\08@231833 by Denny Esterline

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> > I've got about 500 leftover 47 uF caps and maybe 50 22 uF caps, are
> > these tolerable or will they cause problems.
>
> Different MAX232 variants have different cap specs, but none of them that
> large as I recall.  Note that an output short circuit puts a lot more
stress
> on the charge pump pass elements when large caps are used.  I sortof
> remember 1uF is the upper limit.  See data sheet.
>

That's the issue, the data sheet doesn't say. To quote the datasheet for
the DS14C232 (national's equivilant)

External capacitor connection pins. Recommended capacitor:
1.0 5F (6.3V). Capacitor value should be larger than
1 5F.

And the best the Maxim Datasheet says is:

the MAX232E and MAX241E require 15F
capacitors, although in all cases capacitors up to 105F
can be used without harm.
Driving Multiple Receivers

But then it goes on to say:

If in doubt, use capacitors with a larger (e.g., 2x) nominal value.

So basicly an upper limit is not clearly specified.
-Denny

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2004\08\09@000818 by Dwayne Reid

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At 11:23 AM 8/3/2004, Denny Esterline wrote:

>External capacitor connection pins. Recommended capacitor:
>1.0 5F (6.3V). Capacitor value should be larger than
>1 5F.
>
>And the best the Maxim Datasheet says is:
>
>the MAX232E and MAX241E require 15F
>capacitors, although in all cases capacitors up to 105F

The capacitor values above don't make sense.  I'm guessing that the "5" is
supposed to be a mu symbol and the encoding got stripped somewhere along
the way.

Might be safer to just use lower case "u".  I mean: a 105 Farad cap is
rather large for a max232 charge pump <grin>.

dwayne

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2004\08\09@062612 by Denny Esterline

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You are, of course, correct. That should have been a "u", but the weirder
part is that my "sent items" folder says this message was sent six days
ago.


BTW, I didn't "intend" for the "u" to be anything special, I just copied
and pasted from the datasheet....

-Denny


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is
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