casio fx-20

this model is the missing link between the older machines like the fx-10, which had an incomplete function set, to the more mainstream fx-29 and fx-39 style models.

it really is quite a nice little calculator, much smaller than you would expect. it fits neatly in one hand. you can almost use it with the same thumb. this palm-sized style reminds me of the hp 20 series, like the hp-25. when pocket sized really meant it.

quaintly, it has the small zero digit of early casio machines like fx-10 and the memory 8r. like the older machines, this one operates only in degrees and also hms->h is provided but not the other way.

interestingly, natural log shares the same button as x^y. clearly x^y is implemented by continuation from ln by a multiply and e^x. not a bad way to save a button assuming you wont be testing y for integer.

improving on the fx-10 are trig inverses; arcsin, arccos and arctan, a memory (mc, mr, m+), change sign and, at last, proper exponential number representation. ie. x*10^y. displayed as usual with a 2 digit exponent. curiously, the exponent can only range to +/-39 inc. missing features are; 1/x, pi, and the antique a^n operation.

some thought has gone into the keyboard layout, the x^y and ln sharing feature and the two = buttons, one is simple = and the other =M+. when you're not using the memory, you can hit either button making quick calculations quicker.

calculation speed is an improvement over previous models, rather than flicker and twitch for some time, only the right most digit flickers during scientific calculation and not for too long.

the machine only works to eight digits with no guard. trigs display 7 digits, e^x, six and ln, 8. basic calculations and square root are also 8. you cant seem to enter more than 6 digits with an exponent because after the sixth digit is entered, exp is ignored.

trigs stop at the magic 1440 degrees, except for cosine. for some reason, cos accepts input up to 1530. a bug? maybe but the results returned are correct.