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Sharp EL-204

   October 1976 Display technology:  LED
New price:   Display size:  8
Size:  5.0" x 3.0" x 0.90"
 128 x 75 x 23 mm3
   
Weight:  4.1 ounces, 117 grams Serial No:  6311589Y
Batteries:  9V Date of manufacture:  mth 11 year 1976
AC-Adapter:   Origin of manufacture:  Japan
Precision:  8 Integrated circuits:  TMS0956 (KGSΔ7642)
Memories:  1    
Program steps:   Courtesy of:  Ken H. Meine
    Download manual: (EU: 5.4M Bytes)

At first glance looks this early EL-204 calculator like a typical Sharp design of the Seventies but sports a red LED display instead the more common green Vacuum Fluorescent Display (VFD) known from many similar products of this timeframe.

Dismantling the featured EL-204 manufactured in November 1976 in Japan reveals a clean design based on a single-sided printed circuit board (PCB) for the main electronics and a double-sided PCB for the keyboard and powered by a disposable 9 Volts battery.

Removing the PCBs from the calculator housing divulges an unexpected surprise! The EL-204 makes use of a Texas Instruments TMS0956 single-chip calculator circuit based on the TMS1000 Microcomputer family. The TMS0950 architecture was introduced in 1975 with the TI-1200 and TI-1250 and soon replaced with the TMS0970, one of the most successful calculator chip designs.

While the TI-1250 uses a 6x4 layout of the keyboard, changed the EL-204 the layout into a 5x5 matrix adding the sqr(X) function. Reverse-engineering of the EL-204 calculator demonstrates that the TMS0956 actually is scanning a 7x4 keyboard matrix, a previously unknown feature of the TMS0950 series.

The display module manufactured by Sharp of the EL-204 uses nine small LED chips bonded on a substrate with an additional magnifying lens to enlarge the digits.


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

Joerg Woerner, November 25, 2022. No reprints without written permission.