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Sharp EL-8106

This Sharp EL-8106 caught our attention when we started to research the earliest single-chip calculator circuits developed and manufactured by NEC (Nippon Electric Company) Corporation of Japan. NEC introduced in December 1973 with the µPD277 their first "true" single-chip calculator circuit with a 2-key Memory and it was an instant success. We know dozens of calculator designs using the µPD277 from companies like Brother, Citizen, General, Kovac, Miida, Sanyo, Sharp, Silver Reed, Toho Tsusho and many of their brand label products.

We started to look into the µPD277 with the MBO de Luxe III (Version 1) calculator, the middle child of a product line centered around NEC chips.

Sharp obviously decided with the EL-8106 calculator to drop two features of the µPD277, both the square root key and the constant switch are missing on its keyboard layout.

Dismantling the featured Sharp EL-8106 calculator manufactured in April 1975 in Korea reveals a very sturdy design using a metal plate to support the keyboard assembly, an extra frame to hold the display in place, a rather large transformer for its DC/DC converter and even gold plated traces for the printed circuit board (PCB). All electronics of the EL-8106 is placed on one side of a double-sided PCB while the other side forms the contacts for the keyboard. The display is soldered directly to the PCB and three wires connect the electronics to the 3 AA-sized batteries and the connector for an external power supply. Learn how to decipher the 58017464 Date code hidden in the serial number of the pictured calculator.

The PCB is centered around a µPD277 single-chip calculator circuit manufactured by NEC and the few other remaining components on the PCB are mainly used to generate the different supply voltages for the µPD277 and Vacuum Fluorescent Display (VFD) and to bias the anodes and grids of the display with respect to its filament.

On our quest to better understand the application of the µPD277, we decided here at the Datamath Calculator Museum to give the featured calculator a full "Teardown Treatment" and share our findings accordingly.

Calculating Unit: Shortly after introducing the µPD277, it was soon complimented with the µPD276, adding various improvements:

Product Managers at NEC created a textbook example of portfolio management, when they introduced in September 1974 the µPD940 Series and in February 1975 the µPD946 Series to expand their offerings. The µPD940 was placed slightly below the µPD277, missing for example a Memory Function but adding with the [PI] key a Convenience Function and trading the [K] switch for Auto Constant. The µPD946 was placed in the price-over-performance chart over the µPD277, adding a 5-key Memory with Auto Summation and additional Convenience Functions. The next step in the playbook was the introduction of the µPD941, technically and from the manufacturing costs 100% identical to the µPD940 but stripped with some lines of software of the [√x] [PI] keys, a simple measure to protect the pricing of the µPD940 during the Calculator Wars. The µPD277 was eventually phased out and replaced with the µPD278 based on the µPD946:

Display: The featured Sharp EL-8106 calculator manufactured in April 1975 makes use of an 9-Digit low-voltage VFD manufactured by Futaba and known as Type 9-CT-04, soldered with its 19 wires directly to the Main-PCB.

Display Driver: The term "low-voltage" Vacuum Fluorescent Display might be misleading when used together with a calculator powered by three 1.5 Volt batteries. Common VFDs used with portable electronic calculators are usually operated around 30 Volts, significantly higher than the 10 to 15 Volts operating voltage of single-chip calculator circuits used in the 1970s. While the first generation of Texas Instruments TMS0100 single-chip calculator circuits lacked any display drivers and left the choice of display technology to their customers, focused the second generation products mainly on Light-Emitting Diode (LED) technology. In or around 1974, most Western calculator designs still relied on rather expensive LED technology but Japanese companies like Casio, Sanyo, Sharp and Toshiba started to leverage the lower manufacturing costs of VFDs, instead. Texas Instruments introduced in 1974 consequently with the TMS0850 their first product series focused on battery operated VFD calculators and modified the integrated segment and digit output drivers to withstand up to -35 Volts. NEC on the other hand entered the marked of single-chip calculator circuits in 1973/1974 and focused immediately on compatibility with VFDs. The µPD277 chips are manufactured in PMOS technology, meaning the output transistors are "high-side" switching and the most positive voltage of the chip is labeled V_SS for 0 Volt, all other voltages in the calculator are consequently negative with respect to V_SS. Multiplexed low-voltage VFDs need a voltage difference between its filament and the grids and anodes of the numbers of around 30 Volts to light up and to avoid "ghosting" while scanning, the deactivated grids and anodes should be slightly lower than the filament voltage. An elegant and very common solution is found with this Sharp EL-8106 calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (100k Ohm) to around -24 Volts, the filament is biased to around -22 Volts (Zener Diode) and the µPD277 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Sharp EL-8106 makes use of the internal clock oscillator of the µPD277 single-chip calculator circuit, we identified a resistor with 820 kOhm connected between Pin 28 (CLK/R_EXT, C_EXT) of the µPD277 and the negative V_GG power supply line and a capacitor with 82 pF connected to V_SS, resulting in a clock frequency of about 32 kHz.

Power Supply: The Sharp EL-8106 calculator is powered with three disposable AA-sized 1.5 Volt batteries or an external 4.5 Volt power adapter and uses a complex DC/DC converter to generate a total of four voltages:

We measured the operating current of the featured Sharp EL-8106 calculator for two different cases:

Calculating the power consumption at 4.5 Volts for the Sharp EL-8106 results in about 280 mW displaying a '0.' and about 310 mW with all segments but the minus sign illuminated. A very interesting result, a Canon LE-84 calculator with a LED display and using four disposable 1.5 Volt Alkaline batteries and a DC/DC converter for its TMS0801 chip, clocks in at around 100 mW displaying a '0.' and 320 mW with all segments lit; showing both an advantage and disadvantage of LED-based calculators versus their VFD-based counterparts:

Keyboard: The keyboard assembly of the Sharp EL-8106 uses a sandwich design centered around a metal plate screwed into the front part of the calculator housing to accommodate up to 22 spring-loaded plastic keys pushing small conductive pills mounted in a large silicone rubber membrane against contacts etched on a double-sided PCB. The EL-8106 leaves one of the 22 key position empty for a total of 21 keys.

The µPD277 single-chip calculator circuit uses not only its 9 digit driver outputs D1 to D9 to scan the keyboard, it even includes a 10^th output D0 to accommodate up to 30 keys in a 10*3 keyboard matrix. The layout of the keyboard assembly of the featured Sharp EL-8106 calculator shows consequently an arrangement with 10 keyboard scan lines and 3 keyboard return lines. The sliding power switch is soldered directly to the PCB.

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

© Joerg Woerner, April 19, 2025. No reprints without written permission.