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Nippon Calculating Machine Co., Ltd. (Busicom) Model 120-DM Junior Desktop Calculator


Image Courtesy Dentaku Museum


The Nippon Calculating Machine Co., Ltd. (NCM) Busicom Junior, Model 120DM is a desktop AC-powered electronic calculator that was made in two different versions, one that utilized an extremely efficient logic design that utilized only 22 MOS small and medium-scale integrated circuits. The second design packed the logic of all of the 22 integrated circuits into a single MOS/LSI (Large Scale Integration) IC manufactured by fledgeling US chip maker Mostek under contract to Nippon Calculating Machine Co., Ltd.
The Mostek MK 6010 Calculator-on-a-Chip (Note NCM Logo on Chip Lid)

The chip that Mostek did the layout and fabrication for NCM was the first commercial "Calculator on a Chip", and was designated the MK-6010. The logic design of the MK-6010 was created by NCM, and was essentially the logic for the first version of the 120DM that was implemented with small and medium-scale logic. The MK-6010 combined 360 logic gates and 160 flip-flops totalling 2,100 MOS transistors. The MK-6010 was supplied exclusively to NCM initially, but later, due to financial stresses, NCM released Mostek from the exclusivity arrangement in return for compensation from Mostek. The MK6010 continued to be produced for NCM/Busicom, but Mostek created a new part number for the chip that was used for general sale of the chips, which was MK5010. The MK5010 was identical to the MK6010 except for a different number on the package to designate the non-proprietary version of the chip.

In appearance, the difference between the two versions of the Busicom Junior 120DM is that the first version was a bit thicker (approx. 1/2-inch) than the second version because the circuitry for the first version took three circuit boards stacked above each other versus a single circuit board for the second version. There are a number of known OEM customers of NCM that marketed the same calculators under their own brand name, including Sanwa Precisa in Japan, National Cash Register (NCR) in the US, and Elwro (Wrocławskie Zakłady Elektroniczne) in Poland. There may be others out there that have not yet been discovered.

All of the calculators have a very similar appearance with only variances in color and badging. The Old Calculator Museum is looking for any of the machines as listed below. If you have one of these calculators, working or not, please get in touch with the museum by clicking on the EMail button in the menu bar at the top of the page.

- NCM Busicom 120DM Junior (Version I)
- NCM Busicom 120DM Junior (Version II)
- Sanwa Precisa M-12 (Similar to Busicom Jr. Version I)
- Sanwa Precisa Sandiken SG-12 (Similar to Busicom Jr. Version I)
- NCR Model 18-15 (Similar to Busicom Jr. Version I)
- NCR Model 18-16 (Similar to Busicom Jr. Version II)
- Elwro (Wrocławskie Zakłady Elektroniczne, Poland) 105L (Similar to Busicom Jr. Version II)