Lloyd's Accumatic 30 (Model EH-9036, Type 255D)

The Accumatic 30 calculator sold by Lloyd's Electronics, Inc., Compton, California caught our attention when we were looking into the first single-chip calculator circuits manufactured by Sharp Electronics Corporation in Japan, just to discover that this model number was used between 1974 and 1976 with at least 12 different calculator designs sporting 4 different calculator brains.

Dismantling the featured Lloyd's Accumatic 30 (EH-9036, Type 255D) calculator assembled by an unknown Original Equipment Manufacturer (OEM) in May 1975 in Japan reveals a very compact design based on a single-sided printed circuit board (PCB) for the main electronics, a single-sided PCB for the keyboard and powered by four disposable 1.5 Volts batteries or an external power adapter.

The Main-PCB is centered around HD36290 single-chip calculator circuit manufactured by Hitachi and the few other remaining components on the PCB are mainly used to generate the different supply voltages for the HD36290 and Vacuum Fluorescent Display (VFD) and to bias the anodes and grids of the display with respect to its filament.

To gain some knowledge about the differences between the various single-chip calculator circuits used with the Lloyd's Accumatic 30, we decided here at the Datamath Calculator Museum to give the featured calculator a full "Teardown Treatment" and share our findings accordingly.

Calculating Unit: The HD36290 located in the featured calculator could be called the successor of the HD3633 known from the Accumatic 30 (EH-9036, Type 255C) and hampered by its "hard-wired" and buggy logic implementation. The HD36290 offers more flexibility, better algorithm and higher functionality than its predecessor due to switching to programmable ROMs for the calculator logic. It features an integrated clock oscillator and both its segment and digit output drivers are interfacing directly with low-voltage VFDs up to 35 Volts.

Display: The featured Lloyd's Accumatic 30 (EH-9036, Type 255G) calculator manufactured in May 1975 makes use of an 9-Digit low-voltage VFD manufactured by Futaba and known as Type 9-CT-08, a noticeable change to the 8-Digit display and additional small red LED as Minus "dot" of the original Accumatic 30. The display is 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 four 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. Hitachi on the other hand entered the marked of single-chip calculator circuits in 1973 and focused immediately on compatibility with VFDs. The HD36290 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 Accumatic 30 calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (150k Ohm) to around -28 Volts, the filament is biased to around -26 Volts (2.0k Ohm resistor) and the HD36290 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Lloyd's Accumatic 30 makes use of the internal clock oscillator of the HD36290 single-chip calculator circuit, we identified a resistor with 180k Ohm connected between Pin 25 (R_EXT) of the HD36290 and the negative V_GG power supply line, resulting in a clock frequency of about 188 kHz.

Power Supply: The Lloyd's Accumatic 30 calculator is powered with four disposable AA-sized 1.5 Volt batteries or an external 6 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 Lloyd's Accumatic 30 calculator for two different cases:

Calculating the power consumption at 6 Volts for the Lloyd's Accumatic 30 results in about 190 mW displaying a '0.' and about 250 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 Lloyd's Accumatic 30 with the Date code 50.6.05 was manufactured by GICO in September 1975 and uses 19 spring-supported plastic keys pushing small fingers on stamped sheet-metal pieces against contacts etched on a single-sided phenolic PCB.

The keyboard module is connected with 15 pins to the Main-PCB, but only 13 pins are used (9 keyboard scan lines, 2 keyboard return lines, 2 contacts for the [C] key).

Here at the Datamath Calculator Museum we use the DCM-50A Platform to Characterize and Reverse-engineer Single-chip Calculator Circuits. Many designs of electronic calculators do not use all features of their calculator brains and it would be difficult to unleash the full potential of the calculator chips in these cases. Additionally are electronic calculators "closed systems" with limited flexibility to measure signals, change voltages or clock frequencies, provide additional input keys or even change the display technology or specifications additional digits. Core idea of the DCM-50A is providing a generic platform to access all features of a single-chip calculator circuit and with the DCM-50A (PLAYGROUND) we increased the scope from Texas Instruments products to offerings from their competitors in the 1970s, namely AMI, Cal-Tex, Commodore/MOS Technology, Electronic Arrays, General Instrument, Hitachi, Litronix, Matsushita, Mitsubishi, Mostek, National Semiconductor, NEC, Omron, RFT, Rockwell, Sharp, Toshiba, and Western Digital.

The Accumatic 30 sold by Lloyd's mid of the Seventies went through many redesigns within its lifecycle of around 2 years and we started to look into the differences of the four different single-chip calculator circuits located in at least 12 different design iterations. Learn more about our observations here.

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

© Joerg Woerner, November 12, 2024. No reprints without written permission.