Photo of Casio fx-3000 and fx-115MS scientific calculator. An uncle bought me the 3000 back in 1977. Was already gaga over sci calcs back then. It still works but I haven't used it in decades. Though not noticeable in the pic with the 115 (because we're looking head-on through yellow plastic filter) the LCD is already showing its age and deteriorating. Actually it's fared better than my Casio fx-502P whose LCD went kaput years ago. The 3000's power switch had been giving me problems and had to take out its board and clean both the switch and board contacts.
I now use the 6-year old 115 exclusively. It probably has twice the number of functions. The engineering units (nano, micro, kilo, etc) and decimal/binary/hex functions make life easier. And being able to go back and edit the formula/equation and plug in different values for the variables is just indispensable.
As for speed, the two are truly generations apart. A "u" appears as the leftmost character in the 3000 to alert the user that the microcontroller is still running the numbers through its ALU (arithmetic logic unit). Even something as simple as 1+1 takes a quarter of a second. It's that slow. 69! takes 3.3sec. The 115 on the other hand spits out the factorial result in the blink of an eye, literally. Given that both calcs cannot handle or display any number greater than or equal to 10^100, 69 is the highest possible number for factorials for both.
The equation shown on the display of the 115 is (2π*1k*1µ)-1. This of course is the formula for the cutoff frequency of a simple low pass filter with a 1kohm resistor and 1µF cap. Being able to enter "M" for mega, "k" for kilo, "n" for nano, "p" for pico, etc.comes in very handy. They save me the trouble of keying in values in scientific notation.
Back side of the calculator. The sticker is original. Could be the serial number of the unit. You might be able to make out the printing at the bottom. Part of it says "Made in Japan." All Casios were back then. The 115MS on the other hand is churned out in China. Thumbs down.
Without the front panel you can clearly see the poor state of the LCD.
The microcontroller is a NEC D895G. Googled but can't find any datasheet for it.
This is the side of the board that greets you when you take off the back cover.