Here's advice to those considering a used calculator: Reconsider the TI-92 with respect to cost, your true needs, and functionality.

A number of reviews and websites note the differences between the TI-92 and the TI-92 Plus, the latter being pretty equivalent to the TI-89 in terms of its software and functionality and the Voyage 200 being more similar physically, as well. Most advise against the purchase of the '92 in favor of the '92 Plus. If you really understand the differences, you may safely go against this advice.

My comments are based on my own use of the TI-92 in college math classes since it came out and also of the Plus, the TI-89, and the Voyage 200 when they came out. We had two "baskets" of about 20 TI-92's each, half of which were upgraded to Plusses, for use in classes. I have used these calculators in teaching linear algebra classes for about 10 years and prefer them in a classroom setting to the big, computer-based software packages like Maple and Mathematica for just about everything except fancy or color graphics. Moreover, students who don't have or who don't want to lug around laptops have found them useful enough for just about any math at the undergraduate level.

I'm translator of an excellent French book on the '92 family of calculators containing lots of programs up to the graduate level of mathematics. I'm prepared to support the arguments here with examples in just about any application, from basic math education to engineering level applications.

Sometimes, less is more, and always it is possible to get good answers and understanding from a modest technology about which one is informed than from a more powerful technology which may be daunting to use or to master. While there have been complaints about ease of use of the '92, its learning curve is far superior to that of any of the math software packages and generally will build upon school experiences with calculators.

This note only differs from others in these ways:

1) I feel that the TI-92 is superior to the TI-86 in every way except size. It allows more flexible symbolic programming and editing, it has a QWERTY keyboard with few quirks, it has built-in programs and functions not duplicated on many other calculators, it has better graphics, and it may be projected on a screen with a special LCD projector which may be found around math departments in schools and colleges. The floating point arithmetic is good to 13D. The TI-86, based on a different processor and different code, may be faster in some applications, but that rarely is an issue with a calculator.

The chief difference between the math software here and that on the TI-86, for example, is that here you may solve a*x^2+b*x+c=0 for x without declaring the values of a, b, and c: you get the quadratic formula. Or, you can find the symbolic solution to differentiation, integration, and differential equations problems in terms of general coefficients.

On most calculators, solving x^2+1=0 goes nowhere or gives you an approximation. On all the '92 family, you would get "x = i or x = -i".

On this calculator, you may get the derivative of sin(x^2+e^(-(x*cos(x)))) as a "formula," while you can only deal "at a point" numerically and approximately on most calculators. Similarly, if you try to integrate a formula, this calculator will usually get an "exact" answer in terms of "x". If this calculator can't get an exact symbolic answer (up to an arbitrary constant), you may then shift to a numerical method to get a good approximation on a given interval, the standard approach for most advanced calculators.

2) You _will_ need a manual to get the most from it, but if you are only interested in algebra, calculus, differential equations, and linear algebra the basic machine and its menus will likely be enough without copying programs from other sources or doing any programming. Manuals in PDF form are available by reference to ticalc.org or education.ti.com. There are still folks around who can explain any contradictions you find while looking at TI-92 Plus documentation. Originally, the docs for the Plus were in an additional, thin manual to be used in concert with the one for the original '92.

In that original manual, there are 18 applications given in detail in Chapter 19. These include some simple problems analyzed in detail but also include detailed programs and discussions for Finding the Minimum Surface Area of a Parallelipiped, Decomposing a Rational Function (useful in differential equations with Laplace transforms or complex variables), Studying the Flight of a Baseball, Visualizing Complex Zeros of a Cubic Polynomial, Creating a Trisection Macro in Geometry, Finding Rational, Real, and Complex Factors, A Simple Function for Finding Eigenvalues (exactly, rather than by approximation), and interfacing with the many sensors that are available for the TI Calculator Based Laboratory. You won't find many of these, at least in this depth and with the ease of programming, on other calculators, even new ones.

You can translate just about any FORTRAN or BASIC program almost directly into the TI BASIC language of this calculator and there is a fairly comfortable programming environment with ample trace capabilities.

The CAS (Computer Algebra System) software came from the folks who wrote Derive, one of the first math software systems with real power. TI has since bought that company and is the distributor of that computer-based software. Early in the life of this calculator, Derive's creator reputedly actually won a programming standoff in a head-on competition with Mathematica, Maple, and others.

3) This machine is based on the Motorola 68000 processor, the chip that was in the original Macintosh. You can get at the innards sufficiently to program it in assembly, though that will require time at ticalc.org to get the details and to make contact with some gurus of the machine. The development software available from TI for the newer calculators will likely be enough if you are an experienced assembly level programmer. There's more at TICalc.org. If you read French, German, or Italian there are even more resources elsewhere online.

4) Despite the above enthusiasm, you shouldn't pay much for a TI-92 compared to a TI-92 Plus or Venture 200 or TI-89. The latter have upgradeable flash firmware and features have been added which increase their value and, as noted in some reviews and advice columns, render some of their programs unusable on the older machine.

There is a hack which allows you to overclock the processor if you think it's too slow. The processor runs at about 10 MHZ and can at least be doubled in speed.

At an educational conference some years ago, I saw a presentation by a CS grad student from South America who described and demonstrated a peer-to-peer network composed of TI-92 calculators. There are assembly programs for doing gray-scale graphics, clever computer games, and complex control projects. This "calculator" is actually a full-scale small computer to those willing to explore.

5) What can go wrong? Well, you might find a program in a book or magazine which doesn't work exactly as advertised on your TI-92 because it was written for a TI-92 Plus or equivalent. That's unlikely because such published programs are rarely very complex, but usually you will be able to recapture the functionality of the program if you delve into the logic and change the parts where you get run-time errors. The pretty-print BASIC-like program listings make that easy, even if the program is not commented.

As in all keyboard devices, you might get a sticky or stubborn key. The miniature jack used to connect to a computer for exchanging programs can be recalcitrant, and it's not easy to replace. There are a couple of interface cables available, one of which is best for your machine. The other may not work at all. This may take a little research if the cable doesn't come with the machine. It is not a straight through connection, but there's plenty of info online.

It's really convenient to have a friend or a second machine, even if it's a Plus, to compare with when you bump into something inscrutable.

Bottom line:

If you are a high school or college student looking at a future of math through the sophomore college level or if you are a hacker with a special project or lots of time and if you can find a TI-92 calculator for free, or for $20, or $50, or $75 you will likely get your money's worth. Using it you will get educated sufficiently to decide whether to invest in a more expensive version or in fancy CAS software like Mathematica, Maple, MATLAB, Derive, or the like.

If you can afford over $100, don't buy a TI-92, but hang on 'til you can afford one of the newer machines.

Posted as an eBay Guide on March 23, 2008.

# Should I buy a TI-92 vs. the TI-89 or TI-92 Plus?

March 23, 2008

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