This is what you have been waiting for, the hack for the Radio Shack Tape 
Measure (RSTM).  I have successfully hacked the RSTM to my 6.270 board 
with interrupt driven routines and a small amount of C code.  I would 
like to first thank Thomas Rowley and Kevin Ross for their help with my 
coding problems.  Thanks a lot guys.

I will be putting a copy of this text, code, and gifs of the sonar with the 
hack on cher.media.mit.edu.

I do have one problem that I can't explain.  It good for me, but bad for 
people who want to replicate it.  I shorted something on my board that 
fixed a whole lot of my problems with the way the board works.  I do know 
where I shorted things, but I can't promise success in replicating it if 
you try.  I will go into more detail on this towards the end of the text.

There is a big problem with the board if you can't get the fix I did to 
work.  The boards problem is that is fires at least two pulses to get a match 
in distance.  If you are moving or robot or you just want it to stop firing 
after one pulse so you know it will only fire one, I can't help you yet.

I'll get to the hardware hack now.  First of all unscrew the case and 
squeeze the sides of the case to take it apart.  Once apart you can 
desolder the board from the speaker, the white circle on the bottom 
case.  The speaker does not need to be connected for the board to 
function.  You can also desolder the the power leads from the battery pack.  
This depends on your specific application.  You can also desolder the 
transducer if you desire longer leads.

To fire a pulse I attached a wire where the keyboard header is.  I did this
on the solder side of the board.  The pin is the fifth one up from the
bottom.  The return pulse comes from the red wire J2.  Do not disconnect
the wire at either end.  You must solder a wire on the solder side of the 
board.  The return pulse goes high to a certain amount of time, which is 
depended on the distance the ultrasonic pulse traveled.  This next 
section depends on whether you can get my problem to work or not.  A wire 
is necessary to determine whether the board has turned itself off after 
the RSTM timeout period.  If you attach a wire to resistor R16 this will 
give you a digital reading of whether the board is on or off.  You use 
this to determine whether the software needs to turn on the board.

My method of creating a digital output on the 6.270 board without using a 
motor port was to connect a 74LS74 Dual D Flip-Flop to select line S2 and 
to the data bus at D0.  I wire wrapped the small circuit on a piece of 
bread board that fits in the battery pack.  You then connect the wire 
that fires the pulse to the circuit and little bit of power and bingo 
instant digital output.  My software takes advantage of this by writing 
to address 0x5000 with a 1 to fire a pulse.

My Interactive-C binary file is the interrupt code used to read the 
amount of time the pulse is high.  I use the input capture feature of the 
68HC11 to handle this.  The digital input 0 is where the wire that comes
from J2 goes.  The wire that detects whether the board is on or off can be
connected to any of the other digital ports, but the code is written for
port 1.  The code is commented, so this is pretty self explanitory.  My IC
code is also fairly self explanitory.

As for my little problem.  I wish I could explain it to all of you, but 
at this point I have no answer for why this worked.  A little history on 
what I was attempting to do.  The following day after the Seattle Robotic 
Society's meeting I decided to figure out a way to get the board to only 
fire one pulse, this is a big problem with the board.  I was connected my 
digital logic probe up and decided to take a reading at a pad next to 
capacitor C2 when things changed drastically.  I told the computer to 
fire a pulse with my old code and nothing worked.  So, I told it to fire 
again and then again.  Finally on the third try I heard it fire.  IC 
returned one funky value but I knew things were still working.  This is 
when I decided to do each section of code step by step.  I would try to 
make it fire a pulse once and nothing would happen.  So, I attempted 
again and it worked, with only one pulse.  I was very excited.  This is 
were I rewrote my IC program to take advantage of my finding.  I had 
already spent one whole weekend trying to get things to work like this, 
and tried everything I knew how to do.  One set back with this method is 
that the LCD screen no longer works.  For my application this was no big 
deal.  Thomas Rowley is getting schematics which could put some light on 
what I did.

This is my first really successful hack of anything, and I don't have much 
knowledge of analog circuits, so this is quite an accomplishment for me.  I 
can answer questions people might have but as I said, the knowledge of 
analog circuits is not there so I don't know what everything does on the 
board.  I am also quite open to suggestions!

I do suggest getting the gif pictures if you can.  I have put pointed out 
the exact positions of special interest on them.

Hope you are as successful as I was at getting this to work!  I know I am 
really happy to get mine to work.