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'[PIC]: Another problem with RS232 - PIC16F873'
2004\10\27@094622 by Massimo Gaggero

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Hi everybody,
a thread has not been closed on RS232 that I am on beginning an other!

I have a pic 16F873 that sends through its USART module (19200, 8N1) the
"M" & "G" characters (my initials).
USART output is sent to a MAX232 but what I have on my terminal (Windows
HyperTerminal) is only few "M" & "G" between many garbage,
and I have to press continuously "enter" to see the output in the
terminal (I have also tried minicom).

Does anyone have an idea where is the problem?

A thing that makes me suspicious is that the capacitors of the MAX 232
(all except the 100 nF between Vdd & Vss) have the value of 1 uF
(as wrote in the datasheet for a plain Max232CP, not A). But in many
schematics I have seen capacitors of the value of 10 uF.

May be this?

Thank you,
   Massimo.

--
 ____  ____  ____  _  _  
/ ___)| __ \/ ___)/ /| | Massimo Gaggero (spam_OUTmaxTakeThisOuTspamcrs4.it)
| (___ |    /\___ \\__  | Net & System Management
\____)|_|\_\(____/   |_| Phone: +39 070 9250 329


____________________________________________

2004\10\27@103118 by Wouter van Ooijen

face picon face
> I have a pic 16F873 that sends through its USART module
> (19200, 8N1) the
> "M" & "G" characters (my initials).
> USART output is sent to a MAX232 but what I have on my
> terminal (Windows
> HyperTerminal) is only few "M" & "G" between many garbage,
> and I have to press continuously "enter" to see the output in the
> terminal (I have also tried minicom).
>
> Does anyone have an idea where is the problem?

- Do you send full speed? If so insert a wait between each character.
- Did you connect the ground?

> A thing that makes me suspicious is that the capacitors of
> the MAX 232
> (all except the 100 nF between Vdd & Vss) have the value of 1 uF
> (as wrote in the datasheet for a plain Max232CP, not A). But in many
> schematics I have seen capacitors of the value of 10 uF.

don't worry about this

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


____________________________________________

2004\10\27@103903 by Rob Young

picon face
> Hi everybody,
> a thread has not been closed on RS232 that I am on beginning an other!
>
> I have a pic 16F873 that sends through its USART module (19200, 8N1) the
> "M" & "G" characters (my initials).
> USART output is sent to a MAX232 but what I have on my terminal (Windows
> HyperTerminal) is only few "M" & "G" between many garbage,
> and I have to press continuously "enter" to see the output in the terminal
> (I have also tried minicom).
>
> Does anyone have an idea where is the problem?
>
> A thing that makes me suspicious is that the capacitors of the MAX 232
> (all except the 100 nF between Vdd & Vss) have the value of 1 uF
> (as wrote in the datasheet for a plain Max232CP, not A). But in many
> schematics I have seen capacitors of the value of 10 uF.

Find the data sheet for the EXACT Max232 chip you are using (there are
several "clones") and check what it says you should be using for that chip,
your power rails and serial bit speed.  Look at the voltages produced on the
charge-pump pins and confirm they are "in-spec" for your specific chip.

Get you hands on an oscilloscope and look at what the TX line is doing on
both sides of the MAX232 chip.  Eventhough RS232 is supposed to be "OK" with
signal transitions of less than +/-5V, some older PCs aren't happy unless
they get +/-7V or greater on their RX pin.

If you can't get a scope, you can try pulling the digital side of the MAX232
chip HIGH or LOW and measure with a DMM instead.  Doesn't tell you as much
as looking with a 'scope but might help.

By the way, if this discussion turns into a debugging session for the MAX232
and not the PIC, then we should probably change the topic tag from PIC to
EE.

Rob Young
.....rwyoungKILLspamspam@spam@ieee.org

____________________________________________

2004\10\27@113234 by Peter Moreton

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I saw a similar problem when the PCB had been wired without +5v to the
MAX232. The '232 sometimes worked, if data was being sent quickly. Somehow,
the MAX232 was generating a tiny voltage on the VCC pin. (I measured 1.5v on
VCC of the '232)

I don't think your problem will be caused by having 1uf caps instead of 10uf
on the charge pumps. I use a MAX232CSE part with 1uf caps all the time.
Obviously, it's best to use the correct part as specified in the datasheet,
but I don't think this particular part is critical for short RS232 runs.

Peter Moreton




{Original Message removed}

2004\10\27@125542 by Vinicius Hartmann

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The datasheet is always a better reference. To be sure the chip is
working try checking the voltage levels in the output.

When the input is 0 it must be more than 5 volt positive.
When the input is 1 it must be more than 5 volt negative.

Also make sure you have less than 3% error in the baud rate for the
transmitter. More than this will cause garbage.

Lucas Hartmann

____________________________________________

2004\10\28@092422 by alan smith

picon face
Another choice is....the PIC, and the software.
Whenever I test for comm, sending the good ol AA and
55 patterns so I can verify nothing is being reversed
or otherwise corruputed.

OH...I doubt its the PIC hardware.  MAX232 is too easy
to mess up, so I would look at either baud rates or
the program.


               
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2004\10\28@105402 by an=20Mart=EDn=20Semegone?=

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Hi Massimo,
I hope you are doing well. About your problem, here are some comments. First of all, I used 100nf ( .1) caps for the MAX232 charge pump in my applications. So 1uf will work well at 19200. The 100nf caps will work much better in applications with 115200bps for example. It offer many advantages in the slew rate transitions.
The MAX 232 is a quite simple chip, and if you follow the data sheets, it works very well.
Then, you can check many things. First of all, the value you write in the BAUDRATE register, and the cristal you use for your application.
Then if you are using the Hiperterminal or minicom, hide the "local eco characters" from your terminal. If you do so, you will hide the Enter, or any other key you write to the terminal.
Another interesting thing to check is the handshaking mode. Perhaps, it will be useful to begin setting "No Handshaking" or disable the Handshaking mode.
I hope you solve your problem soon.
Kind Regards,
Martín.
Instituto Argentino de Radioastronomía.


Peter Moreton <peter.moretonspamKILLspamvirgin.net> wrote:
I saw a similar problem when the PCB had been wired without +5v to the
MAX232. The '232 sometimes worked, if data was being sent quickly. Somehow,
the MAX232 was generating a tiny voltage on the VCC pin. (I measured 1.5v on
VCC of the '232)

I don't think your problem will be caused by having 1uf caps instead of 10uf
on the charge pumps. I use a MAX232CSE part with 1uf caps all the time.
Obviously, it's best to use the correct part as specified in the datasheet,
but I don't think this particular part is critical for short RS232 runs.

Peter Moreton




{Original Message removed}

2004\10\29@023603 by Massimo Gaggero

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Thanks a lot to everybody for your suggestions.
I have found. It was the Signal Ground: I have found that the signal
ground (DB9, pin5) was broken, I made a new soldering and now
it works.
Thank you,
   Massimo.

--
 ____  ____  ____  _  _  
/ ___)| __ \/ ___)/ /| | Massimo Gaggero (.....maxKILLspamspam.....crs4.it)
| (___ |    /\___ \\__  | Net & System Management
\____)|_|\_\(____/   |_| Phone: +39 070 9250 329


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