Javier wrote:
>
> How can I emulate the open collector  in the keypad ??
> I don4t fully understand, why it4s better to send 04s and not 14s

This technique is used only if you will not put any external component
(except for switches) on your keypad.  To make the software simple,
diodes and some pullup/down resistors are needed.

To do without external components, the port (PORTB) must be programmed
to enable it's weak pull-ups.  That is why sending 0 is more economical
because no resistors are needed.

The port output registers must also be put to low and only work on the
TRISB registers when pulling low the rows on each pass.

setup:
        bcf     STATUS,RP0              ;bank0
        clrf    PORTB                   ;set output registers low
        bsf     STATUS,RP0              ;bank1
        movlw   0xFF                    ;tristate
        movwf   TRISB                   ;
        bcf     OPTION,RBPU             ;enable weak pull-ups
        ...
        ...

        (in keyread routine)
        ...
        movlw   B'11111110'             ;scan row
        ...
        movlw   B'11111101'
        ...
        movlw   B'11111011'
        ...
        movlw   B'11110111'
        ...
        bsf     STATUS,RP0              ;bank1
        movwf   TRISB                   ;enable particular row
        andlw   0x0F                    ;mask row
        movwf   key                     ;save in key
        bcf     STATUS,RP0              ;bank0
        movf    PORTB,w                 ;read port
        andlw   0xF0                    ;mask column
        iorwf   key                     ;combine with row


regards,
Reggie