Texas Instruments TI-55-II

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: Numeric display  
New price:   Display color: Black  
    Display technology: Liquid crystal display 
Size: 6"×3"×½" Display size: 8+2 digits
Weight: 4 oz    
    Entry method: Algebraic with precedence 
Batteries: 2×"LR44" button cell Advanced functions: Trig Exp Hyp Lreg Cmem Units 
External power:   Memory functions: +/-/×/÷/^ 
    Programming model: Partially merged keystroke 
Precision: 11 digits Program functions:  
Memories: 8(1) numbers Program display: Keycode display  
Program memory: 56 program steps Program editing: Overwrite capability  
Chipset:   Forensic result: 9.114640577  

ti55ii.jpg (25499 bytes)Yet another of Texas Instrument's "almost programmable" calculators, the TI-55-II is obviously a successor of the TI-55. The LED display was replaced with an LCD panel and buttons were added that made programming somewhat easier.

Interestingly, the TI-55-II had less memory than the TI-55, even though it was more flexibly partitioned. Whereas the TI-55 had a total memory equivalent to 10 storage registers (8 addressable registers, 2 of which overlapped 16 out of the calculator's 32 program steps), the TI-55-II has the equivalent of 8 registers, although it is possible to convert as many as 7 of these into 56 steps of program memory.

Another difference is the behavior of the RST function in program mode. On the TI-55 it resets the program counter to 00, but program execution doesn't stop, which allows for the creation of simple looping constructs. On the TI-55-II execution does stop, so similar loops are not possible (or at least, I haven't found a way to create them in the absence of a manual.)

This severely limits the utility of this calculator, despite the presence of advanced functions such as a numerical integral, which evaluates the integral of the function represented by the keycode sequence in program memory. Without loops, one cannot write a factorial program, for instance; the limited size of the program memory makes it impossible to enter complex expressions such as those used for the Gamma function. Instead, here is a version of Stirling's formula that provides a reasonable approximation of the factorial for arguments greater than 0.5. (For very small arguments, the function quickly diverges, whereas for 0 or negative arguments, it results in an error.)  The program is small enough to leave room for 4 storage registers, of which one is used by the program itself:

00  61    STO
01  00    0
02  65    ×
03  02    2
04  65    ×
05  91    π
06  95    =
07  13    √
08  65    ×
09  71    RCL
10  00    0
11  45    yx
12  71    RCL
13  00    0
14  55    ÷
15  71    RCL
16  00    0
17  41    INV
18  44    lnx
19  65    ×
20  53    (
21  71    RCL
22  00    0
23  17    1/x
24  55    ÷
25  01    1
26  02    2
27  85    +
28  01    1
29  95    =
30  22    RST