The Colex 812-R calculator, introduced in 1975, is a robust and feature-rich device that stands out for its unique design and functional capabilities. Manufactured by Colex in Hong Kong, this calculator is encased in a glossy white plastic shell with a black smooth plastic front, reminiscent of the Commodore PET computer in terms of its key design. The calculator’s dimensions are approximately 93mm x 150mm x 32mm, and it weighs 196 grams without batteries.
Powered by 3.0V DC using two AA batteries, the Colex 812-R also supports an adaptor/charger through a socket on the top side. The battery compartment, interestingly, can accommodate four AA batteries, but only two terminals are active, with the others blocked by foam. The full recharge time, as indicated on the back panel, is eight hours.
The calculator features an 8-digit green Vacuum Fluorescent Display (VFD) with a ninth digit for negative indication. The keys have a medium-long travel with a soft click and strong bounce-back, adding to the tactile experience. The display is inset with a green plastic filter and white printed text for the number positions, enhancing readability.
In terms of functionality, the Colex 812-R offers standard four functions along with percentage, square root, reciprocal, and a five-function memory. The calculator’s logic is noteworthy, with excellent recovery capabilities and unique features such as negative root recovery, which is quite unusual for calculators of its time.
Logic
- The (C) key cancels an entry, and a second press clears the entire calculator.
- Input overflow is not suppressed; inputting a ninth digit causes the number to flash, which is recoverable using (C).
- Negative numbers are displayed with a “-” in the immediate left digit, traveling into the far left (ninth) digit, allowing full eight-digit negative numbers.
- Automatic constant is available on all four functions.
- Overflow results in a flashing display, recoverable using (C).
- Divide by zero shows zero flashing, recoverable using (C), but it suffers from a bug: keying in (9)(/)(0)(=) results in a flashing “0.”, and keying (C)(2) shows “0.2”. This bug does not occur with (0)(1/X), where recovery using (C) is perfect.
- Negative square roots are not allowed and result in the original number flashing, recoverable using (C).
- Memory store is not indicated; users must remember the stored value.
- Overflow in memory stores the result, shifted.
Summary
The Colex 812-R calculator is a large-sized, well-designed device with attractive keys that, unfortunately, do not tend to survive well. Its logic is robust with excellent recovery features, only let down by a strange divide-by-zero bug. The calculator’s ability to handle negative root recovery is particularly noteworthy. It shares similar functions and internal components with the Busicom/Colex 812-SL, making it a notable addition to the calculator market of the 1970s.
