0.4 The bugs  (Page 4/4)

But that solution wasn’t permanent. The line still periodically shut down as a result of the bug. Then, finally, the bug was found and quietly fixed. It seems that the state machine on the speech synthesis chip had a 22 state counter. The counter was simply a 32 bit counter that reset at the count of 22. Unfortunately, the 10 undefined states were not designed to drive the counter back to zero. Once in the unknown part of the counter, it would go in an undefined loop with garble. So, the bug went away. And, no, to this day, I still can’t explain why the four capacitors solved the problem. Nor do I care to.

Obviously, this was the hardest bug of all to fix. It nearly cost me my job at TI. But the personal gain was a significant increase in my confidence and reputation within the company.

Broken keys

There is nothing worse than keys falling off of the Speak N Spell. And it was always the same keys, the R, S and T (these three keys are not only in the area of the keyboard with the problem, but are also keys that get used the most). In figure 2 I’ve included a picture from the back of the front case showing the three keys missing on an early unit. This was finally resolved by eliminating the buttons by using a Mylar flat keyboard instead. Although a more reliable and cheaper solution, the Mylar keyboard never was, in my opinion, as good of a solution as the button keyboard. Now for the story of how it happened.

Richard Chang, the person who did the mechanical design for the Speak N Spell was new to TI. As an aside, he later became the CEO of a major corporation in China and I am proud to claim to be his first boss. He did a great job on the design and the plastic case fit together on the first attempt to assemble it.

Before I continue on to the issue with the keyboards, I’d like to tell another story about the mechanical design of the case. Richard made and error in the design by not specifying the various tolerances on the drawings. Without this specification, the assumed tolerance was 0.01 inches. This was far too tight of a tolerance for a plastic design. When we received the parts, the initial quality inspection found virtually every measurement to be wrong, even though all of the parts snapped together exactly as they were designed to do. But the 0.01 inch tolerance specification generated a very long list of variances. Each of which needed to have an Engineering Change Notice (ECN) written and approved. Fortunately it didn’t slow down the project, but created a great deal of busy work for Richard.

Now, back to the story of the broken keys. To make the keyboard work without having to design a new keyboard component required us to use two high volume calculator keyboards butted to each other (see Figure 1). This made the plastic buttons offset from the optimal place to press on the keyboard to make the keys actuate. So, for a few of the keys, when the button was pushed, the key hinge was improperly stressed to electrically actuate the key. Given enough key pushes, the key would literally fall off. Fortunately, once found, the problem was resolved with a bit of creativity and knowledge of the issue. Sometime during the second year of production the button keys were replaced by a membrane keyboard which permanently make this bug go away. But it was at a cost of loosing the tactile feel of the keyboard.

How to determine the date of manufacture of a speak n spell

Now, the good news is, if you want to buy a Speak N Spell, in the after market, from the first year of production, the fact that the product has button keys and there are keys missing, is a dead give away. Yes, they are still available on Ebay.

Next, for an important way to actually determine when the Speak N Spell you own was manufactured. The answer is on the back case where the serial number and date code have been scribed. Here is how the date code works. It is a seven character sequence under the serial number. It has the form of three letters and four numbers:

AAANNNN

The three letters represent the location of manufacture. The first year of manufacture was in Midland, Texas. That would make the three letters MTA for Midland, Texas, America. The four numbers that follow give the week and year of manufacture. We began manufacture of the Speak N Spell in August of 1978. So the first two numbers should be somewhere between 30 to 52. The last two digits should be 78. See Figure 3 for an example of the date code.