Miida MC840 Electronic Calculator

The Miida MC840 is a basic four-function home/office calculator from the early 1970's. The MC840 is a 'clone' of a ground-breaking calculator, the Omron 800, which shattered the price barrier for a four-function desktop calculator when it was introduced April of 1971. After a while of producing the 800 on its own, Omron made OEM agreements with vendors such as Miida and Adler to provide the 'guts' for their calculators. This allowed other manufacturers to distinguish themselves with their own cabinet designs, using the low-cost insides developed by Omron to keep their prices down. Based on the date codes on the IC's inside the machine, this machine was likely made in late 1972. The machine uses an 8-digit vacuum-fluorescent display panel (rather than discrete VF tubes), and has an additional 9th display position which contains a negative sign for display of negative numbers, and a "10⁸" notation to indicate overflow (with the display showing an approximate result if multiplied by 100000000). The MC840 is housed in a sleek, wedge- shaped case with highly contrasting display panel and keys with a polar-white case color. The machine was definitely a styling statement for its time.

Miida 840 With Back Cover Removed

The calculator is built upon a three chip calculator chipset made by Omron (which, at the time this machine was made, appears to have been a second source for Hitachi's calculator chips), a Japanese electronics manufacturer. The chipset has part numbers HD32104, HD32105, and HD32106. Date codes on all of these chips are 2E. The machine also uses Toshiba VF display driver chips, part number TM4352. The MC840 uses a magnetic reed switch keyboard, and a basic transistor-regulated power supply, with an internal voltage selection switch. The main board connects to the VF display panel via 14-pin IC-sockets on wire pigtails from the main board which plug onto the back of the display panel.

Miida 840 Circuit Board

The 840 is a floating point machine, with the unusual distinction of having 'left to right' digit entry. Many calculators of the day used 'right to left' digit entry. The calculator offers a constant function which operates very differently than most. The "K" key is a push-on/push-off key. When depressed, the "+=" and "-+" keys lose their functionalty. Only the multiply and divide keys function perform any function when the constant function is active. The constant is entered on the keyboard, then the "K" key is depressed, then any number which you wish to be multiplied or divided by the constant is entered, then the "X" or "divide" key pressed to calculate the result. Any other numbers which are desired to be worked with the constant can then be entered, followed by "X" or "divide", for as long as the constant is desired. If a new constant is desired, the "K" key must be deactivated, the "C" key pressed to clear the constant, a new constant entered, and the "K" key depressed again. This is an extremely unusual method for providing a constant function, and in some ways, rather confusing.