Werner,

 As you probably know, CMOS parts use FET's.  One of the
 terminals on an FET is the DRAIN.  This is, power supply
 wise, likened to the COLLECTOR on a BJT.  An open DRAIN
 connection is just that.  The DRAIN is not connected to
 the power source.  This is so that you can connect a resistor
 to this pin and then to the power supply, so that you can
 have a wired 'OR' connection.  Just put a pullup resistor
 from 'RA4' to Vdd, and everything should work fine.  I would
 use from 1K to 5K ohms for this pullup.

                                          Regards,

                                            Jim


On Tue, 21 March 2000, Werner Soekoe wrote:

>
> Hi
>
> I wonder if anyone could adivse me. I made a PIC16F84 circuit using all 13
> I/O pins to drive 13 LEDs, but the LED connected to RA4 doesn't light up. It
> appears that the pin does provide a signal, though. When I read up some more
> in the PIC16F84 datasheet, I came accross a paragraph which said that all RA
> pins are CMOS drivers, except RA4 which is an "open-drain" output. Is this
> why it can't deliver current? What does the "open-drain" mean?
>
> Thanks
>
> Werner Soekoe
> wsoekoe@global.co.za

jim@jpes.com