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'[EE:] More Max232 problems :('
2004\08\03@200037 by Anthony Toft

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This morning I got the PC to talk to the PIC, now I am trying to get PIC
to PC working and it isn't.

The Max232 is generating a waveform that looks like the incoming wave
shape wise, but the outgoing (ie being generated by the Max) is 0v and
12v, instead of the normal 12v and -12v levels. This is measured at the
DB9. Pins 2 and 6 on the Max are approx 8 and -8.

I have tried different cables in case it was a short, but no help.

Once again I am asking for your learned opinions.

Anthony
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2004\08\03@200656 by Anthony Toft

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> I have tried different cables in case it was a short, but no help.

BTW with the cable unplugged, the voltage is +-8v

Thanks
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2004\08\03@201112 by Spehro Pefhany

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At 08:00 PM 8/3/2004 -0400, you wrote:
>This morning I got the PC to talk to the PIC, now I am trying to get PIC
>to PC working and it isn't.
>
>The Max232 is generating a waveform that looks like the incoming wave
>shape wise, but the outgoing (ie being generated by the Max) is 0v and
>12v, instead of the normal 12v and -12v levels. This is measured at the
>DB9. Pins 2 and 6 on the Max are approx 8 and -8.

Are you sure you have the proper pin (5) on the DB-9 grounded to your
PIC circuit ground?

Also try connecting 6 to 7 (only), and connecting 4,1 and 6 together (only)
at the DB-9.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
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2004\08\03@202537 by Martin Klingensmith

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Anthony Toft wrote:
>>I have tried different cables in case it was a short, but no help.
>
>
> BTW with the cable unplugged, the voltage is +-8v
>
> Thanks
> --
> Anthony Toft <@spam@toftatKILLspamspamcowshed.8m.com>

Hi Anthony,
if the voltage is correct with the wires disconnected then you likely
have the wrong pins connected on the DB9
It is easy to get them switched around accidentally. Have a look at this:
<www.zytrax.com/tech/layer_1/cables/tech_rs232.htm>
You need pins 2 and 3 on the DB9 connector.

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2004\08\03@210733 by Anthony Toft

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> if the voltage is correct with the wires disconnected then you likely
> have the wrong pins connected on the DB9

This is not the problem as the PIC receives data from the PIC correctly,
it is the PIC to PC where the levels are not good.

The only thing I can think of is the App note says to use a 1uF bypass
and I have a 0.1uF, could this be the problem? The current electrical
environment I think is fairly clean, and it _is_ working for the Rx.

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2004\08\03@211938 by Martin Klingensmith

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Anthony Toft wrote:
>>if the voltage is correct with the wires disconnected then you likely
>>have the wrong pins connected on the DB9
>
>
> This is not the problem as the PIC receives data from the PIC correctly,
> it is the PIC to PC where the levels are not good.
>
> The only thing I can think of is the App note says to use a 1uF bypass
> and I have a 0.1uF, could this be the problem? The current electrical
> environment I think is fairly clean, and it _is_ working for the Rx.

Try it and see

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2004\08\03@232845 by Anthony Toft

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> > and I have a 0.1uF, could this be the problem? The current electrical
> > environment I think is fairly clean, and it _is_ working for the Rx.
>
> Try it and see

I have, it made no difference, plugged in I have +1.6v and -11v

I also tried turning the cables around and that made it so the PIC
couldn't Rx data either.

This is ticking me off now, the really irritating part about it is that
Wouter uses a very similar set up in his wisp628 and _that_ works!

I am coming to the conclusion that my max might be fried in some wierd
and wonderful(?) way.
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2004\08\04@002449 by Brent Brown

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> > > and I have a 0.1uF, could this be the problem? The current
> > > electrical environment I think is fairly clean, and it _is_
> > > working for the Rx.
> >
> > Try it and see
>
> I have, it made no difference, plugged in I have +1.6v and -11v
>
> I also tried turning the cables around and that made it so the PIC
> couldn't Rx data either.
>
> This is ticking me off now, the really irritating part about it is
> that Wouter uses a very similar set up in his wisp628 and _that_
> works!
>
> I am coming to the conclusion that my max might be fried in some wierd
> and wonderful(?) way. -- Anthony Toft <EraseMEtoftatspamcowshed.8m.com>

Jumping right in here.. might I suggest connecting the wires one by one to
determine exactly what is dragging down the rail in question?

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2004\08\04@080421 by Anthony Toft

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> Jumping right in here.. might I suggest connecting the wires one by one to
> determine exactly what is dragging down the rail in question?

T1out is connected directly to the DB9, with the DB9 disconnected it
swing +-10v or so, with the DB9 connected it swings +1,-10v but
sometimes -1,+10v this seems to be dependant on the PC transmitting
data.

I am going to replace the Max232 and see if the (invisible) magic smoke
has come out, it's the only thing I can think of :(


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2004\08\04@080422 by Olin Lathrop

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Martin Klingensmith wrote:
> You need pins 2 and 3 on the DB9 connector.

More specifically, the outgoing signal from pin 14 of the MAX232 should be
connected to pin 2 of the DB-9, the incoming signal from pin 3 of the DB-9
to pin 13 of the MAX232, and pin5 of the DB-9 to ground.

Double check the pin pattern of the DB-9.  The numbers are usually stamped
in tiny letters next to each pin.  You may have flipped everything around by
accident.


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(978) 742-9014, http://www.embedinc.com

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2004\08\04@080423 by Olin Lathrop

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Anthony Toft wrote:
> BTW with the cable unplugged, the voltage is +-8v

OK, so you're MAX232 is working correctly.  Something is wrong on the other
end of the cable.


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2004\08\04@080424 by Olin Lathrop

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> The Max232 is generating a waveform that looks like the incoming wave
> shape wise, but the outgoing (ie being generated by the Max) is 0v and
> 12v, instead of the normal 12v and -12v levels.

Actually it should be about -8 to +8.

> This is measured at the DB9.

Is anything connected to the DB-9 at the time.  If yes, disconnect and
measure again.  If not, then there is something wrong with your MAX232
circuit.

> Pins 2 and 6 on the Max are approx 8 and -8.

That's about right.  This shows that the MAX232 has power and ground, and
that the charge pumps are working.  You may just have a busted chip.


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2004\08\04@174424 by Brent Brown

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> > Jumping right in here.. might I suggest connecting the wires one by
> > one to determine exactly what is dragging down the rail in question?
>
> T1out is connected directly to the DB9, with the DB9 disconnected it
> swing +-10v or so, with the DB9 connected it swings +1,-10v but
> sometimes -1,+10v this seems to be dependant on the PC transmitting
> data.
>
> I am going to replace the Max232 and see if the (invisible) magic
> smoke has come out, it's the only thing I can think of :(

Maybe, but could well be incorrect wiring with one or more MAX232 outputs
connected to a PC COM port output. Give us a full diagram/list of pinouts if
need be.

My most memorable MAX232 problem was my confusing of the TTL and
RS232 sides of the schematic symbol! The design rule checker showed no
problems (inputs are input etc) and a netlist check showed the PCB had no
errors (matched the schematic perfectly :-)), so my mistake ended up on the
PCBs. Lots of fine wires to cross pins on a SO16 narrow package!

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2004\08\04@234455 by Anthony Toft

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> Maybe, but could well be incorrect wiring with one or more MAX232 outputs
> connected to a PC COM port output. Give us a full diagram/list of pinouts if
> need be.

The schematic is located at

http://cowshed.8m.com/car/inputs.html

Also there is the board layout I put together with eagle, note that the
pins marked "serial" are currently hooked up in a standard fashion, the
12v and 0v coming from an external power supply, in the final
application, all the power will come from the remote PC.

Eagle shows the board to pass DRC with no errors, the MAX232 schematic
is copied verbatim from the appropriate TI datasheet, and it corresponds
with how Wouter did it on his WISP628. The PIC can receive the serial
data, and the Noritake responds properly.

I have yet to get the chance to swap out the MAX, I was busy elsewhere
this evening
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2004\08\05@001203 by Brent Brown

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> > Maybe, but could well be incorrect wiring with one or more MAX232
> > outputs connected to a PC COM port output. Give us a full
> > diagram/list of pinouts if need be.
>
> The schematic is located at
>
> http://cowshed.8m.com/car/inputs.html
>
> Also there is the board layout I put together with eagle, note that
> the pins marked "serial" are currently hooked up in a standard
> fashion, the 12v and 0v coming from an external power supply, in the
> final application, all the power will come from the remote PC.

The "standard fashion" bit needs to be explained. A PC COM port typically
has a D9 male connector and we just need to see which pin numbers
connect to where in your circuit. I don't mean to patronise but from end to
end what you should have is the following:

MAX232 pin 14 (TXD) ..............PC D9 pin 2 (RXD)
MAX232 pin 13 (RXD) .............PC D9 pin 3 (TXD)
MAX232 pin 15 (GND)..............PC D9 pin 5 (GND)

Regards, Brent.

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