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'[EE]: max232 problems'
2002\07\17@110741 by Robert Berkes

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I want to let my pic communicate to the pc. I hooked everything including
the max232acpe chip up as described by 'Care and Feeding of PIC16C74
(FACT003)'. (using several 0.1uF capacitors and a 10 Ohm resistor)
When I try to communicate from the pc to the pic, I can see on an
oscilloscope that the max232 is receiving the data from the pc properly
(between 0 - 15 V), but the max232's output (to the pic) stays always
high at 5 volts.
I checked the setup several times and also tried it with three different
max232.


Thanks,

Rob

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2002\07\17@114247 by Tan Chun Chiek

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have u enabled RX in RCSTA and also set the RX pin as input?


----- Original Message -----
From: "Robert Berkes" <spam_OUTrberkesTakeThisOuTspamINI.PHYS.ETHZ.CH>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Wednesday, July 17, 2002 10:57 PM
Subject: [EE]: max232 problems


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2002\07\17@121142 by Olin Lathrop

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> When I try to communicate from the pc to the pic, I can see on an
> oscilloscope that the max232 is receiving the data from the pc properly
> (between 0 - 15 V),

Something already doesn't seem right here.  The signal from the PC should be
symmetric about 0, usually around +-8V with the idle state low.  This sounds
like a ground is not hooked up properly.  Make sure pin 5 on the PC
connector is connected to the PIC circuit ground.


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2002\07\17@163316 by Peter Grey

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At 04:57 PM 17/07/2002 +0200, you wrote:

According to the data sheet the max232 should have 1.0uF caps not 0.1uF.
Just yesterday I was migrating from a max202 (5 volts) to a max3232 (3.3
volts). Like you I had a signal from the PC when I pressed a key in
hyperterminal but nothing from the max3232. In the data sheet it shows that
the cap on pin 2 can be either connected to Vcc or Vss. I had it connected
to Vss. With the max202 it is connected to Vcc. When I connected the cap to
Vcc all was OK. The data sheet is WRONG.

I hope it helps.

Peter
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2002\07\18@051341 by Robert Berkes

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What am I missing? Port 5 of the serial connector is hooked up to the
pic's ground. But the signals I got from the PC are still between 0 and
15V.
Following setup:
       Pin 1, 9 floating
       Pin 2 max232 transmit
       Pin 3 max232 receive
       Pin 4 connected to Pin 6
       Pin 5 connected to Gnd with a 10 Ohm resistor
       Pin 7 connected to Pin 8

Sorry for that kind of a stupid question but I've got no idea what could
be wrong.

Thanks

Rob


On Wed, 17 Jul 2002, Olin Lathrop wrote:

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2002\07\18@054342 by Mircea Chiriciuc

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If almoust the same I use to do the communication (I don't think there are
many ways to wire it up), but I don't wire pin4 to pin6 and pin7to pin8.
Are you shure you have set up the PIC pin connected to receive as an input.
If not and Pin=0 then that is why you don't have nothing getting out of the
max232. Simplest way of checking that if to leave that pin floating (no
connection between MAX and PIC and look with the scope. If there is output
from the MAX there is your mistake. otherwise is something wrong with the
MAX. Schematics would help more. Try this first.

Mircea Chiriciuc

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2002\07\18@061038 by Jinx

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part 1 186 bytes content-type:text/plain; (decoded 7bit)

>         Pin 2 max232 transmit
>         Pin 3 max232 receive

This what I always use for PIC to PC. Numbers in parentheses
are the other pair


part 2 2411 bytes content-type:image/gif; (decode)


part 3 131 bytes
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2002\07\18@064911 by Brent Brown

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Jinx wrote:
>         Pin 2 max232 transmit
> >         Pin 3 max232 receive
>
> This what I always use for PIC to PC. Numbers in parentheses
> are the other pair

Jinx, I wasn't following the thread but your diagram caught my eye.
You show opposite connections of what is written above, is that what
you meant? Sorry if I'm missing something.

With a D9 female connector on the PIC board it's a good idea to wire
it as if you were going to plug it in directly to a PC serial port
(D9 male), or use a straight through cable, in which case:-

PIC board D9F pin 2 = MAX232 transmit, pin 7 or 14
PIC board D9F pin 3 = MAX232 receive, pin 8 or 13

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2002\07\18@072112 by Jinx

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>  >         Pin 2 max232 transmit
> > >         Pin 3 max232 receive

> Jinx, I wasn't following the thread but your diagram caught my eye.
> You show opposite connections of what is written above, is that
> what you meant? Sorry if I'm missing something

The connections as written above are Robert Berkes - leaving them
in the reply was a subtle hint. A little too subtle, it appears ;-)

I have to admit if I didn't have a diagram I'd need to look it up
in a book every time. Anyone else got blind spots like that, things
you just can't remember ?

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2002\07\18@072950 by Olin Lathrop

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> What am I missing? Port 5 of the serial connector is hooked up to the
> pic's ground. But the signals I got from the PC are still between 0 and
> 15V.
> Following setup:
>         Pin 1, 9 floating
>         Pin 2 max232 transmit
>         Pin 3 max232 receive
>         Pin 4 connected to Pin 6
>         Pin 5 connected to Gnd with a 10 Ohm resistor
>         Pin 7 connected to Pin 8
>
> Sorry for that kind of a stupid question but I've got no idea what could
> be wrong.

Take this one step at a time.  It doesn't make sense to proceed until you
see the proper levels coming from the PC.

Disconnect everything from the PC.  Get rid of those wires you have between
pins of the PC connector too.  Connect the scope ground to pin 5 and look at
the signal on pin 3.  It should be between -5 and -15 volts, usually
around -8V.

Now set up the PC to continuously emit a character.  I do this by going into
HyperTerm and rigging a weight to hold down one of the keyboard keys.  Make
sure that flow control is set to NONE.  Otherwise set the start bits, stop
bits, and parity the way you will be using them.  This also assumes you have
keyboard auto-repeat enabled, although this is done in Windows, not in
HyperTerm.  Set up the scope to trigger on the rising edge.

You should now see a digital signal on the scope with levels roughly -8V and
+8V.  The idle state will be low, the start bit high and the stop bit low.
A 1 bit is high and a 0 low.

Proceed by hooking things up incrementally until this signal in its various
forms makes it to the PIC UART input pin.  At that point it will be 0 to 5V
with the idle state high.


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2002\07\18@073405 by Brent Brown

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> >  >         Pin 2 max232 transmit
> > > >         Pin 3 max232 receive
>
> > Jinx, I wasn't following the thread but your diagram caught my eye.
> > You show opposite connections of what is written above, is that what
> > you meant? Sorry if I'm missing something
>
> The connections as written above are Robert Berkes - leaving them in
> the reply was a subtle hint. A little too subtle, it appears ;-)
>
> I have to admit if I didn't have a diagram I'd need to look it up in a
> book every time. Anyone else got blind spots like that, things you
> just can't remember ?

I don't remember these things like I used to so looked at a diagram.
Good little boys should be alseep by this time of night...

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2002\07\18@080121 by Olin Lathrop

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part 1 1080 bytes content-type:text/plain; (decoded 7bit)

> >         Pin 2 max232 transmit
> >         Pin 3 max232 receive
>
> This what I always use for PIC to PC. Numbers in parentheses
> are the other pair

Are you really sure!?  I think you've got PC pins 2 and 3 flipped in the
schematic, although you defined them correctly above.  Pin 2 receives data
into the PC and pin 3 transmits data from the PC.  Perhaps your schematic is
intended to work with a cable that flips 2 and 3.  Most PCs have male DB-9
connectors, so I like to put female DB-9 connectors on my PIC projects and
wire them so that all you need is a "straight thru" DB-9 extension cord.  I
think of it as the project connector plugging in directly to the PC, except
that the extension cord is required to make is mechanically possible.

Attached is a schmatic of such a project that has been built and works.
Note that this used an ST variant of the MAX232 chip which requires 100nF
caps instead of 1uF.  A +10V supply was also available, so the chip was only
wired up to generate the -10V supply.


part 2 10218 bytes content-type:image/gif; (decode)


part 3 305 bytes

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2002\07\18@080537 by Robert Berkes

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I changed the connections according to Jinx's layout so that
       PIC board D9F pin2 = MAX232 transmit, pin 13
       PIC board D9F pin3 = MAX232 receive, pin 14
and MAX232 is now receiving properly. But the manufacturers
datasheet, the layout (from 'FACT003') and Brent say that
transmit is on pin 14 and receive on pin 13?
I only wonder if transmission will work!

Rob


On Thu, 18 Jul 2002, Brent Brown wrote:

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2002\07\18@081417 by Alan B. Pearce

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>and MAX232 is now receiving properly. But the manufacturers
>datasheet, the layout (from 'FACT003') and Brent say that
>transmit is on pin 14 and receive on pin 13?
>I only wonder if transmission will work!

I suspect you are falling foul of another problem with the "RS232 Standard"
and that is the definition of transmit and receive. I believe the definition
is the signal direction as seen by the Data Terminal Equipment (DTE) which
should be the item with the male pins on the connector. hence the mating
connector the "transmit" signal is the signal coming INTO the equipment. You
will not be the first to hiccup over this, and I doubt you will be the last.

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2002\07\18@082020 by Jinx

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> > >         Pin 2 max232 transmit
> > >         Pin 3 max232 receive
> >
> > This what I always use for PIC to PC. Numbers in parentheses
> > are the other pair
>
> Are you really sure!?

Not any more :-) Told you I've a mental block for cables. When I
labelled the diagram I mistakenly counted pins for a 14 pin IC
on the PCB in front of me, even though I know full well a 232 is
16 pins. At least it's just the diagram that's wrong, the circuit
works with the PSP cable

Pin 7 of the 232 goes to pin 2 of the DB9
Pin 8  of the 232 goes to pin 3 of the DB9

It's been a long week and it ain't over yet

------------------------------------

So my apologies to My Berkes. He should be able to get serial
working with just 3 wires

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2002\07\18@082221 by Jinx

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> I changed the connections according to Jinx's layout so that
>         PIC board D9F pin2 = MAX232 transmit, pin 13
>         PIC board D9F pin3 = MAX232 receive, pin 14
> and MAX232 is now receiving properly.

I'm going to bed before I get in more trouble

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2002\07\18@082852 by Robert Berkes

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Ok I connected everything step by step and took measurements. Everything
works fine until I connect the receiving pin (13) of MAX232 with pin 3 of
the D9. Then the voltage drops to about -19V (from -8.9V).
When connecting pin 14 of MAX232 to pin 3 of D9 then MAX232 works
properly (as it was proposed by Jinx, but as you said he has most probably
pin 2 and pin 3 of D9 mixed).

Does somebody know if I'm using the correct capacitors?
       MAXIM MAX232ACPE chip with 0.1uF Capacitors.
MAXIM say on their datasheet, that I have to use 1uF capacitors for the
MAX232 chip and 0.1uF capacitors for the MAX232A chip. Does MAX232A
include MAX232ACPE?


Thanks again

Rob


On Thu, 18 Jul 2002, Olin Lathrop wrote:

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2002\07\18@084525 by A.J. Tufgar

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> Does MAX232A
> include MAX232ACPE?

Yup.
http://www.seattlerobotics.org/encoder/aug97/cable.html

Aaron

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2002\07\18@092113 by Robert Berkes

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It's finally working, thanks to all!

Rob

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2002\07\22@052436 by Robert Berkes

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I'm testing the pic's usart. I send input from the hyperterminal to the
pic over a MAX232 chip. The pic just returns this data and wants to send
it to the hyperterminal again. Everything looks ok when I measure the
input/ouput from/to the pic wiht an oscilloscope. But as soon as I connect
the output (MAX232 pin 14 or pin 7) to pin 2 of the DB9 (serial connector)
the voltage of the input signal (idle) drops from about -8V to -18V and
there's no output signal anymore.

Any suggestions?

Thanks Robert

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2002\07\22@054337 by Rajiv Thakur

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One suggestion:

1. Short pin 2 and 3 of our Serial port cable coming  from PC. Now Send data
from Hyperterminal. It should bounce back on Hyperterminal. This means your
serial port and PC is fine and the problem is with the PIC/Max232.

2. I suggest that you verify the harware connection critically  including
the polarity connection of  your 10 muf or 1 muf capacitor. If the  circuit
is fine then its a software problem .I have always found Max 232  works when
the harware and sofware is correct.

Rajiv Thakur




{Original Message removed}

2002\07\22@060756 by Robert Berkes

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If I short pin 2 and pin 3 then the idle voltage raises from -9V to 0V and
there's nothing appearing at the hyperterminal? The signal's maximum is
then around 10V. Does this seem like a problem with the serial port?
When I use the serial port to program the pic there's no problem...

Thanks Robert


On Mon, 22 Jul 2002, Rajiv Thakur wrote:

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> {Original Message removed}

2002\07\22@080853 by Rick C.

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Recheck carefully your pin numbers on the DB9 (or DB25). You might be reading it
wrong.
Rick

Robert Berkes wrote:

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> > {Original Message removed}

2002\07\22@085131 by Bill & Pookie

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RS232 is a protocol that is most likely older than
you and has a few things that may cause confusion
in the modern world.  I assume that you are only
using the transmit and receive lines.  If so, make
sure that hyperterm does not use "hardware"
handshake.  Also, is your cable a straight through
cable or is it a "null modem" cable where the
transmit and receive pins are swapped?  Would
advise "ringing out" the cable by doing a
continuity check on the pins you use.

Shorting the pins on the cable is a great test.
Maybe next, remove power from the pic board,
install the cable and short the pins on the max232
chip that is connected to the transmit and receive
pins in the cable.  Again, whatever you transmit
will be returned to hyperterm.

Then the last test might be to program a pic with
the uart pins as just input/output pins and have
the program continuity look at the input pin and
make the output pin the same.  Kind of a software
shorting of pins.  And if you have a led on the
board, have it show the state of the input pin,
that would let you see what logic levels the line
uses.

I don't have information on pins and such, but a
description of what pins the three lines go
through might help others.  pc db(9 or 25) <-> pic
db (9 or 25) <-> max 232 <-> pic, for transmit,
receive and circuit ground.

Bill





{Original Message removed}

2002\07\22@102207 by Robert Berkes

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Thanks the problem is solved. I was as stoopid as to hook up the DB9 as
seen from the backside...

Thanks to all

Robert




On Mon, 22 Jul 2002, Bill & Pookie wrote:

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> {Original Message removed}

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