Bowmar MX25 (Version 1)

Bowmar introduced their first MX20 "Brainchild" more than two years after the famous Bowmar 901B, better knows as Bowmar Brain. Many changes were necessary to reduce manufacturing costs of electronic calculators to achieve some profit with a suggested retail price of $59.95 for the MX20 compared to $240 of the 901B:

When Texas Instruments introduced with the TMS0803 an improved programming of the TMS0801 single-chip calculator circuit and added a percent function to the its basic four-function, Bowmar immediately introduced this MX25 "Brainchild" Here at the Datamath Calculator Museum we refer to the MX20 "Brainchild" as Bowmar's 3^rd Generation Portable Calculator design:

Dismantling this Bowmar MX25 (Version 1) calculator manufactured in July 1974 in Mexico reveals a clean design based on a double-sided printed circuit board (PCB) for the main electronics and a single-sided PCB for the keyboard and powered by three disposable AA-size batteries or an optional rechargeable battery pack.

The Main-PCB of the featured MX25 calculator is centered around a TMS0803 single-chip calculator circuit manufactured by Texas Instruments, two ITT B6249 Digit Driver chips and two ITT B6370 Segment Driver chips soldered with some additional wiring in place of the intended ITT B6248 parts. The remaining components on the PCB are mainly used to generate the necessary supply voltages of the TMS0803 and its clock signal for the internal timing.

On our pursuit of not only to document the differences between the siblings MX20 (Version 1), MX20 (Version 2), MX25 (Version 1) and MX25 (Version 2) but also to understand the rationale of switching the calculator brains from Rockwell to Texas Instruments and finally to General Instrument, we decided here at the Datamath Calculator Museum to give some of Bowmar's 3^rd Generation calculators a full "Teardown Treatment" and share our findings accordingly.

Switching suppliers of key components was very common during the heights of the "Calculator War", maybe to save a few pennies on manufacturing costs or due to supply chain constraints, Canon's Palmtronic LE-83 series went through at least four different manufacturers of LED Displays and the Lloyd's Accumatic 30 used five or six different calculator chips over the course of just two years. But with Bowmar we think that the reasoning might have been different. The Rockwell 15332 chip located in the MX20 (Version 1) was clearly a carry-over from the MX50, basically an update from the Model 905 still using the ancient TMS0103 single-chip calculator circuit from the famous Model 901B. The MX50 was adding a percent function to the feature set of the Model 905 while dropping the odd [+=] and [−=] keys in favor of more common [+], [−] and [=] keys. The MX20 didn't offer the percent function, though. In a time when you could charge a few bucks extra for an additional key on a calculator keyboard, was it reserved for the more expensive MX25. When Texas Instruments introduced in Fall 1973 the TMS0800 Product Family, the focus was clearly on dramatically cutting the manufacturing costs of low-end, battery operated handheld calculators. Precisely what Bowmar was looking for and consequently was the first member of the TMS0800 series, known as TMS0801 designed into the MX20 (Version 2). TI followed soon with the TMS0803 adding the percent function and the MX25 (Version 2) was born. But the story doesn't end here!

When General Instrument introduced the C-500 single-chip calculator circuit based on an "inherited" Pico Electronics design, it was already outdated when it hit the market. It lacked the common scanning of a keyboard matrix and offered no flexibility for the calculator manufacturers to differentiate their products. Product Managers at General Instrument understood the advantages of the newly released TMS0800 chips very well and aimed squarely at it when they introduced in 1974 the C-593. It was advertised as "socket-compatible" with the TMS0803 and the CF-593 was a 100% replacement of the TMS0855. This made the design-in phase of the C-593/CF-593 very easy for the calculator manufacturer but General Instrument had an ace up their sleeve - the C-593 was just one member of a multi-dimensional Product Portfolio ranging from 8-digit four-function calculators to 9-digit scientific calculators using either LED Displays or VF-Displays and operated with 9 Volt or 15 Volt supplies. All in one standardized, TMS0800 compatible pinout. When General Instrument published in 1978 the final Data Catalog featuring Calculator Chips, the Product Family grew to over 30 members in a two-dimensional Portfolio Chart.

General Instrument C/CF-580, C/CF-590 and C/CF-680 Series - Display and Power Supply:

General Instrument C/CF-580, C/CF-680 and C/CF-590 Series - Calculator Function:

The TMS0803 is a member of the TMS0800 family, introduced in 1973/1974 and following the original TMS0100 "Calculator-on-a-chip".

The unusual ITT B6249 display driver located with this Bowmar MX25 manufactured in March 1974 is an early sign of Bowmar's deteriorating relationship with Texas Instrument staring in the 1973/1974 timeframe.

The Optostic R7H-112-9A display module of the Bowmar MX25 uses seven individual GaAsP (Gallium Arsenide Phosphide) Segment LED chips and one GaAsP Decimal Point LED chip per character bonded directly to a double-sided FR4 printed circuit board (PCB) and placing a one-piece red acrylic magnifying protecting lens on top of the assembly with four heat stakes.

Klixon™ is a trademarkof Texas Instruments.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, September 20, 2022. No reprints without written permission.