I think you are right there. I copied this design (used diodes) because I
couldn't be bothered manipulating the TRIS bits, so I'm lazy!

___________________________________________
Wesley Moore
RMIT - BEng/BApp.Sc. 2nd Year

wmoore@cs.rmit.edu.au
http://wmoore.tsx.org/

On Fri, 28 Apr 2000, Michael Rigby-Jones wrote:

>
>
> > -----Original Message-----
> > From: Andrew Seddon [SMTP:seddona@HOTMAIL.COM]
> > Sent: Friday, April 07, 2000 11:14 PM
> > To:   PICLIST@MITVMA.MIT.EDU
> > Subject:      Re: Interfacing TI calc to PIC
> >
> > I think I can get away without the pullups. I say this as the calc has
> > it`s
> > own internal ones. To pull the line high I just turn the relevant pin to
> > an
> > input, e.g high impedence and let the pullups do the rest. This is to
> > avoid
> > the shorting situation.
> >
> > Just to check I put two 10k pullups in but unfortunately it didn`t make
> > any
> > difference.
> >
> > What I want to figure out know is why Sami Khawam has diodes in his PIC
> > connection, web address http://sami.ticalc.org . I can`t see why they
> > would
> > be usefull. Also the homemade parralell link has both resistors and diodes
> > yet has no internal pullup. This has go me mighty confused.
> >
> I suspect that he is using the diodes to effectively turn the pin into an
> open collector/drain. i.e. the pin can pull down, but not source.  Not
> really need IMHO, he could have toggled the corresponding TRIS bit for that
> pin, ensuring that the output latch was set low.  This would also have
> negated the need to use 2 pins per line.  This is just a semi-eductated
> guess though.
>
> Mike
>
>
>