> It is definetely possible to implement your own  serial protocol directly on
> the parallel port because there are 5 input and many output pins on the PC
> parallel port which you can interface to directly.  The outputs of
> the PC printer port is given below.  The signals marked with a - are
> inverted.  The ones who's direction are marked OUT, actually mean
> that you can use them as inputs from your PC side.

Note that [I'll have to see if the 'parallel port FAQ' documents this] on
most parallel-port cards, including the originals, the strobe, select,
auto-lf, and init outputs may all be used as open-collector bidirectional
signals (so the total is five dedicated inputs, eight dedicated outputs,
and four bidirectional).  This can be extremely convenient, esp. for the
RB7 'programming data' signal.  **BUT** some parallel ports, most notably
Toshiba laptops, do NOT support this.  If using these pins in a design
makes things easier, go ahead and do it (it will work on 99% of the PC's
out there) but be sure to DOCUMENT it and provide a workaround.