-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Leonardo De Palo
Sent: Wednesday, August 28, 2002 3:48 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]:16F877 and MAX3232 suppression trouble.
(Dammeyer/Lathrop updated)


Hi PICer friends,


referring to my last EMAIL, and appling the previous Jhon Dammeyer and the
Olin Lathrop suggestion, I have cut/past/manipulated this schematics:



YOU COULD USE A SERIES RESISTOR between the PIC and the Zener!!!
The PORT pin driver will be stressed without it.


PIC out pin ----------|----------------->RX 3.5V device
                      |
                     ___/
                    / Z
                      E
                      N 3.5V
                      E
                      R
                      |
                     GND



PIC RB input <--------------+
with internal               |
pullup acivated             |
                            C
                    BC547    B<---+------TX 3.5V device
                            E     |
                            |     _
                            |     ^
                            |     D1 1n4148
                            |     |
                            +------
                            |
                           GND


but I'm not convinced about the D1 1n4148 because its presence do not permit
at the signal to reach zero level.

NONSENSE!  It ONLY clamps NEGATIVE input Voltages.


What happen if I remove it and stay only
with the BC547?

NOTHING different, you just don't have the rail clamp.

 (I suppose that the junction BE also do not permit to rech
zero level; it is true?)

NO, You might DESTROY the BE junction because AGAIN...
YOU SHOULD USE A SERIES RESISTOR between the TX 3.5V device and the BC547
Base!!!

To have better wiev, I use the SERIN/SEROUT instruction of PICBASICPRO and
the connection is made a 19200bps, when works ;-)


Regards

Leo


> Rereading our EMAILS, I have understood that the problem that I
> have posted was given in wrong way.
>
> A little history can help.
>
> In normal working condition with PC and the device that I have at
> the begining:
>
> PC COM1: TX----->MAX3232-------->RX 3.5V device
>
> PC COM1: RX<-----MAX3232<--------RX 3.5V device
>
> and works PERFECTLY (obviously because is normal working
> conditions)
>
>
>
>
> Using a PIC powered at 5V instead of a PC:
>
> PIC out pin ----->MAX3232-------->RX 3.5 device
>
>
> PIC in  pin <-----MAX3232<--------TX 3.5 device
>
> and works PERFECTLY!
>
>
>
>
> Trying to remove the MAX3232 (It is not a typing error for MAX232, > but
the version for the 3.5V powered device), maintaining the PIC
> powered at 5V:
>
> PIC out pin -------Diode-Diode--------->RX 3.5 device
>
>
> PIC in  pin <------resistor-------------TX 3.5 device
>
> in this condition I have a working condition, (that means the TX
> and RX levels are near to good) but unreliable.
>
>
> I suppose now you have a better view of my problem.
>
>
> Saluti
>
> Leo
>
> --

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