Brother ProCal 310 Calculator
The machine uses Mitsubishi MOS LSI ICs but no date codes can be identified. The Brother corporate history website depicts an identical-looking machine as the “Model 710 with LSI chips” and dates this model to 1970. The back page of the Pro Cal 310 User Manual has a code number of 592-710, perhaps also suggesting that the 310 and 710 models are closely related, if not indeed the same.
The Pro Cal 310’s MSI chips were at the cutting edge of 1970 Japanese IC fabrication technology but the resulting machine is three-function integer-only with no decimal point entry or management and no native division. The instruction manual is explicit about these limits and devotes a page to describing how division may be done by looking up the divisor in a table of reciprocals and using the calculator to multiply by this reciprocal as an integer. The user must keep track of the decimal positions and decimal-adjust the calculator’s answer manually. The manual does not go so far as to supply the table of reciprocals! It is interesting that in 1970 Brother judged that there would be sufficient market interest in a small (but still mains-powered) machine that could not divide and could not handle non-integers.
There is of course a relationship between integer-only operation and lack of division because integer addition, subtraction and multiplication will always give integer results. Division of one integer by another will most often not, and the number of decimal digits in the result is variable. It is very difficult to present the results of division in a usable fashion on a display without a decimal point.
The Model 310 uses arithmetic input style but the user interface is inconsistent and rather poorly laid out. There is a “+=” key, a “X” key which is visually similar and a “-” key which is visually different. Unfortunately, these visual cues do not reflect the manner of operation of each key.
“+=” is used after the first operand is entered, to transfer the operand to working store and to prepare for the second operand. After the second operand, “+=” performs addition and displays the sum while “-” performs subtraction and displays the difference. “-” should probably be marked “-=”.
“X” initiates multiplication after the first operand or performs multiplication and displays the result after the second operand. This should also probably be marked “X=”
Negative numbers are displayed conventionally, with the negative lamp illuminated. The rules of arithmetic are correctly followed for addition, subtraction and multiplication with negative numbers. The “-” key can designate a first or second operand as negative but there is no means to change the sign of a stored number other than formally multiplying by -1.