The Busicom Colex 812-SL is a scientific calculator from 1975. It has a slim, brick-shaped plastic case measuring 77mm x 143mm x 17mm and weighs 172g including the internal rechargeable batteries. The calculator is powered by 4.8V from four N-sized 1.2V Matsushita batteries. It can be recharged using an adapter that plugs into a socket on the top and provides 6V DC.
The 812-SL has a striking design with a two-tone white plastic case and inlaid black plastic trim surrounding the key area. The keys are soft and springy with silver and black printed labels, similar to some Commodore scientific models. It has a flush, green vacuum fluorescent display with room for 8 main digits plus an extra digit for negative numbers and error indicators.
Features are fairly basic for a mid-1970s scientific calculator. It includes the four standard arithmetic functions plus square root, percentages, and a 5-register memory. The logic allows full 8-digit negative numbers and has an automatic constant on all functions. However, it lacks more advanced scientific features like trigonometric or logarithmic functions.
Interestingly, this calculator was made by Colex in Hong Kong but branded as a Busicom model. All paperwork and packaging is labeled Colex, with only a small Busicom sticker on the front. This may indicate unauthorized or unofficial rebranding when the calculator was imported to the UK. The high-quality design and unusual branding make the Busicom Colex 812-SL an intriguing example of early electronic calculators.