Port.92P:

The Story :->
^^^^^^^^^^^^^

For a long time, the TI-92 Link port remained a mystery. Mel Tsai and
myself had spent several hours investigating the port, and countless
others had been doing the same. Then a guy named Bryan Turner used
his program called "Raven" to figure out how the link port works. It
turns out the individual line control IS possible on the TI-92. Bryan
Turner sent a few messages to Mel Tsai, which Mel forwarded to me. So
what you have is a port test program, that allows you to control either
line, or read either line. Though I wrote the program, the method is
entirerly Bryan Turner's, and he deserves the credit.

How it Works
^^^^^^^^^^^^

When the program loads, it sets bit 6 of $60000C, and clears all the
other bits. This bit is the port "master-bit" in that it controls the
port mode. With it on, individual line I/O is possible. With it off,
the port uses some kind of UART. Once this bit is set, it is possible
to read the port status by examining bit 2 or 3 of $60000E, and it
is possible to change the port lines by writing to bit 0 or 1 of this
same address. This was what Bryan Turner found out with his Raven program,
and Mel Tsai and I confirmed it. Because of Bryan Turner's discovery, the
following are now possible:

** An expander for the TI-92!
** Stereo Sound
** An IR link
** An Oscilliscope Program!
** A frequency counter

And many more! If you still don't quite get the method Bryan Turner
discovered, look at 92link.txt. It contains the messages Bryan Turner
sent to Mel Tsai, and which were forwarded to me. Hopefully
they won't mind me including them in the package.

The Program
^^^^^^^^^^^

When the program starts, it sets the master line control bit. It then
gives you the option of reading the port or writing to it. I'm a little
nervous about allowing both at the same time, since if you write a 1 to
both lines and then short them to ground, things could get hairy. Maybe
in a future version I will include this, since it is extremely difficult
to break the link port hardware. In the input mode, the only valid key
is escape. The status of the lines will update on the LCD as you change
their electical states. In output mode, the R and W keys toggle the
red and white wires, respectively. In output mode, escape also works
to exit the program. When the program exits, it writes $8D to $60000C
which resets the port and allows transfers with other calculators
or your PC.

Note that this program is a quick kludge, and is by no means
optimized. But its fast, and it isn't too big. Most importantly, it
works. :-) If you tighten up the code any, please send me the smaller
version and I will release it, giving you credit.

Enjoy! Hopefully, once this program and more importantly Bryan Turner's
discovery get known, we will see some cool programs and some cool hardware
for the TI-92.

I'm already planning an IR link.

Bryan Rittmeyer
bryanr@flash.net