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Commodore
786D
1976

Commodore 776M

Brief History
Commodore, seeing the success of the pocket calculators purchased from Bowmar, began developing their own machines. Its factories were built in the USA, in the UK and later also in Japan and Hong Kong. After the Minuteman Series, it has become clear that assembly can no longer be profitable because of the depressed prices dictated by large semiconductor companies which produces their own calculators (such as Texas Instruments or National Semiconductor). By keeping all the plants operating, they also cut costs by developing new circuits (RBP and GRBP series) in 1975 that could operate directly from a 9V battery, thus significantly reducing the soldering time of printed circuits. Assembling the keyboard was moved to Japan, where cheaper labor was available. So the cost of assembling a four-function model was reduced at the British plant, and the manufactured quantities were increased. 7 and 8-digit calculators (774D, 776M, 784D, 786D) and the first simplified scientific calculator (SR7919) were produced from this year. The housing was made in several colors, the machines can contain memory or not so there were many variations: the second digit of the model number reflected the display capacity, the last letter described availability of memory (represented by M). The price war later forced Commodore to relocate the production of four-function models to Japan and later to Hong Kong, and to concentrate on the production of still profitable scientific and specially designed models in western plants.
Manufacturer:Commodore Business Machines (USA/United Kingdom)
Mfg. date:1975
Size:6,4×13,7×2,4 cm
Weight (ready for operate):116 g
Type:four-function
Capacity:7 digits (input/display)
7 digits (internal precision)
Operating logic:algebraic
CPU:Commodore GRBP-67
Registers:2 standard (with saving the pending operation)
1 memory (without arithmetic)
Features:%calculation with percent Ffloating-point notation
Display:9 digit LED (Bowmar Optostic R7M-042-9B)
Power:9V battery or adaptor

Commodore 776M

PCB of the 776M

Keyboard of the 776M
Detailed Description

Technical Details

The first series were built with GRBP-67 (7-digit machines) or GRBP-89 (8-digit machines) CPU, which needed another chip to connect to the LED display. In addition to the functions available on these calculators, GRBP circuits are capable of register exchange, reciprocal and square calculations.

Later series are made with the 3D-98MT (MPS7560) circuit, which can now be directly connected to the display. This made it possible to mount the chip and LED display on the keyboard printed circuit. This IC was designed for a wide range of calculators, so memory operations can be accessed with a single key M or the standard four-key solution (M +, M-, MR, MT).

Interestingly, the later series of the 786D incorporated the 3D-98MT circuit as well as the 9-digit versions, but the connection to the first digit has been cut, which means that there is not any sign of error or negative sign for 8-digit numbers. Instead of register-exchange key, it had memory key. When the missing connection was restored, this calculator become a full-function 796M model.

The lower-capacity versions were built with lower-capacity displays and, on the other hand, partially defective ones that has errors on leftmost digits only. The machines use a variety of compatible types of displays and drive circuits, so easy to find Bowmar's Optostic displays as well as in-house pieces.

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