Terminated: unknown
It is an 8-digit calculator but it has a 12-digit fluorescent display. The extra digits are used for displaying exponents, a non-zero memory value (a small upside down "u" between numerical values and the exponent) and negative values using the leftmost digit.
It is a quite powerful scientific calculator for its time, with trigonometric functions, logarithms, brackets (2 levels deep), memory (including M× and M÷ operations), angle conversion, and the x! factorial operation.
The four basic functions are automatically repeated when pressing the = key.
The xy function only accepts non-negative values of x.
It does not have a key or function for squaring numbers, but the × = key sequence works for that.
There is a hint at the back with an example on how to calculate 5×5×5: press 5 × × =. The last operation and operand are reused when pressing =.
Another example: to calculate 15÷5 press 1 5 ÷ 3 =.
Now, to divide any other number by 3 also simply type that number followed by pressing = again.
Interesting stuff happens when one does not use the = key during calculations.
An example sequence:
8 ÷ 2
−: Intermediate result: 4.
−: Now subtracts 2 leaving 2.
3 ×: 3 is unexpectedly subtracted leaving -1.
×: Multiplies by 3 leaving -3.
5 +: Actually multiplies by five, leaving -15.
+: Now 5 is added, leaving -10.
+ + +: Adds 5 three more times leaving 5.
3 +: Back to 8.
And so on...
This is probably unintended behaviour and a side effect of its internal
calculation method.
8 ÷ 2
−: Intermediate result: 4.
−: Now subtracts 2 leaving 2.
3 ×: 3 is unexpectedly subtracted leaving -1.
×: Multiplies by 3 leaving -3.
5 +: Actually multiplies by five, leaving -15.
+: Now 5 is added, leaving -10.
+ + +: Adds 5 three more times leaving 5.
3 +: Back to 8.
One thing though, the placement of some of its keys, especially the = key, is awkward.
It has a built-in battery pack, which has been removed from this specimen. It still runs perfectly on a 4.5 V power adapter.
There is a Radio Shack variant on this model, the Radio Shack EC-490 which uses different key colours. Personally I prefer this version.
There is an article about this calculator on the National Museum of American History website here (link validated 2025-12-06).