Bowmar MX20 (Model 90911)

Date of introduction:  December 1973 Display technology:  LED Optostic
New price:  $59.95 (December 1973) Display size:  9
Size:  5.4" x 2.8" x 0.85"
 152 x 71 x 21 mm3
Weight:  4.0 ounces, 112 grams Serial No:  195196
Batteries:  3*AA Date of manufacture:  mth 03 year 1974
AC-Adapter:   Origin of manufacture:  Mexico
Precision:  8 Integrated circuits:  TMS0801, 2*ITT B6249, 2*unknown
Memories:   Displays:  Bowmar Optostic R7H-112-9A
Program steps:   Courtesy of:  Joerg Woerner

The Bowmar MX20 was introduced 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 "Bowmar Brainchild" compared to $240 of the 901B:

The complex and expensive Klixon™ keyboard manufactured by TI was replaced by a much simpler design
The 6 rechargeable batteries of the 901B were replaced by 3 disposable batteries
The expensive LED display with single LED modules was replaced with an Optostic assembly
The first generation TMS0103 single-chip calculator circuit was replaced by a second generation TMS0801

Dismantling this Bowmar MX20 calculator manufactured in March 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.

The main PCB is centered around a TMS0801 single-chip calculator circuit manufactured by Texas Instruments, two ITT B6249 Digit Driver chips and two unklnown Segment Driver chips. The remaining components on the PCB are mainly used to generate the necessary supply voltages of the TMS0801 and its clock signal for the internal timing.

The TMS0801 is a member of the TMS0800 family, introduced in 1973/1974 and following the original TMS0100 "Calculator-on-a-chip".

With the DCM-50A Platform developed to Characterize and Reverse-engineer Single-chip Calculator Circuits we could proof that the TMS0801 program code matches the program code example described in Texas Instruments' Patent Application US3934233A covering the TMS0800 architecture.

The unusual ITT B6249 display driver located with this Bowmar MX20 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 Bomwar MX20 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.

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© Joerg Woerner, September 6, 2022. No reprints without written permission.