Texas Instruments TI-68

Datasheet legend
Ab/c: Fractions calculation
AC: Alternating current
BaseN: Number base calculations
Card: Magnetic card storage
Cmem: Continuous memory
Cond: Conditional execution
Const: Scientific constants
Cplx: Complex number arithmetic
DC: Direct current
Eqlib: Equation library
Exp: Exponential/logarithmic functions
Fin: Financial functions
Grph: Graphing capability
Hyp: Hyperbolic functions
Ind: Indirect addressing
Intg: Numerical integration
Jump: Unconditional jump (GOTO)
Lbl: Program labels
LCD: Liquid Crystal Display
LED: Light-Emitting Diode
Li-ion: Lithium-ion rechargeable battery
Lreg: Linear regression (2-variable statistics)
mA: Milliamperes of current
Mtrx: Matrix support
NiCd: Nickel-Cadmium rechargeable battery
NiMH: Nickel-metal-hydrite rechargeable battery
Prnt: Printer
RTC: Real-time clock
Sdev: Standard deviation (1-variable statistics)
Solv: Equation solver
Subr: Subroutine call capability
Symb: Symbolic computing
Tape: Magnetic tape storage
Trig: Trigonometric functions
Units: Unit conversions
VAC: Volts AC
VDC: Volts DC
Years of production:   Display type: Alphanumeric display  
New price:   Display color: Black  
    Display technology: Liquid crystal display 
Size: 6"×3"×½" Display size: 12 characters
Weight: 4 oz    
    Entry method: Formula entry 
Batteries: 1×"CR-2032" Lithium Advanced functions: Trig Exp Hyp Lreg Solv Intg Cplx Cmem BaseN Units Const 
External power:   Memory functions: +/-/×/÷/^ 
    Programming model: Formula programming 
Precision: 13 digits Program functions:  
Memories: 55(0) numbers Program display: Formula display  
Program memory: 440 program steps Program editing: Formula entry  
Chipset:   Forensic result:  

ti68.jpg (22046 bytes)The TI-68 is a somewhat odd calculator. Texas Instruments produced many "almost programmables"; calculators with a facility to store keystrokes, but a very small program memory, and no program control instructions such as conditional or unconditional branching. The TI-68 qualifies as an "almost programmable" except that its prgram memory is fairly large; a total of 55 storage registers can be converted into approximately 440 program steps, in the form of up to 26 programs. Yet, no branches, no loops, no conditionals, not even a facility to call a second program from within a first (given that each individual program is limited to 79 program steps, this is a severe restriction.)

It is also a very fragile beast, difficult to repair. You almost need a stick of dynamite in order to open the plastic case (I have not been able to open it without causing some structural damage.) Inside, the LCD display is connected to the main circuit board using a piece of flexible circuit that is attached by some adhesive. If this circuit starts to peel off for any reason, the damage is almost impossible to repair.

Because of the limitation of 79 program steps per formula, it is not possible to program the Gamma function into this calculator, at least not without the use of variables for constants (the calculator in this case will inelegantly prompt for the value of each variable used, every time you run the program) or multiple programs (which would need to be called explicitly by the user.) Instead, here is a solution for Stirling's formula:

S=(X+1) yxX×√(2×π×(X+1))÷ e^(X+1)×(1+1÷12÷(X+1))