here is a rather splendid datamath calculator from texas
instruments. the ti-2500 first released in 1972 had six different
versions. this example is version 2 and dates from 1973.
the unit is powered by 4 AA cells inside the unit and is meant to be
used in conjunction with the mains adapter which charges the batteries for
up to 15 hours.
notice the peculiarity of placing the add and subtract buttons down the
right and the multiply and divide across the top. at first, i expected the
unit to operate like a desk calculator (eg 3+2- rather than 3-2-=), but it
doesnt, it operates like any 4 function unit would today.
the power switch is on the left side of the machine and a switch on the
klixon keyboard selects between chain or const operation. const
is a way of activating multiply and divide constants like 2*3=, 4=,
5= in the normal way. a similar selector is available on the