16F877A system and tachometer (FIS version) on the table

[mazoul72]

Hello,

i have a system with a PIC16F877A that is in serial with steering wheel command (RESET / UP / DOWN on B5,B6 et B7)
I am programing to put ground on outputs (RC3 ,RC4 ,RC5 ) but for valid UP/DW/RESET i need transistion from +12V to ground .

Also i need absolutely 12V like FIS send to input (normally) and PIC has only +5V to 3 wires .
How have +12V on RC3 ,RC4 and RC5 outputs command by my program ? (pull-up resistors ??)

Regards.

[aratti]

Simply use optoisolators.

Al.

[mazoul72]

I will have +5v on isolator inputs and +12v on output ?

[mazoul72]

I have
Input pulsup (B5,B6,B7)
Input pulsdw
Input pulsres
and
Output mup (C3,C4,C5)
Output mdw
Output mres
High mup
High mdw
High mres

But it is the problem of 5V , no +12V . I don't understand ,+12V (output FIS) to +5V (output PIC) , it is a BIG problem ??

[mazoul72]

My program works like this :

a detection for RBx cause a different display to FIS Tachometer and for input isolator (output RC5,6,7) cause a +5V and +12V on output isolator .
BUT it is dangerous because a +12V on output isolator cause a failure on FIS tacho input voltage .
I would like only a No intrusive +12V but GROUND ONLY (action with ground input RBx) .
FIS give +12V in normal operation :

[aratti]

If your FIS tacho device doesn,t requires too much current at the inputs the opto is the solution ( remember that the signal is inverted. in other words when pic pin is low the output is high) I have separated the two grounds, but they can be common.

If on the other hand you will require more current at the input then use the second solution which uses two transistors.


Cheers

Al.

[mazoul72]

i put today +12V directly on FIS Tacho input , with harware power failure ...
What is the difference with +12V (10Kohms resistor) ??

Thanks for your answer ...