Royal Model 91K (UA122)

Litton Industries, headquartered in Beverly Hills, California, acquired in 1958 Monroe Calculating Machine Company, an American manufacturer of mechanical calculators. In 1966 Litton acquired Imperial Typewriter Company Ltd, a very successful manufacturer of typewriters and merged it with its own Royal Typewriter division. In 1969 the conglomerate was further growing with the acquisition of Triumph-Adler, the merger of UK Triumph Cycle Company and Adler, a German manufacturer of bicycles, typewriters, sewing machines and calculators. Early in the 1970s, Litton sold, depending on the region, electronic calculators under five different brands: Adler, Imperial, Monroe, Royal and Triumph.

Royal Typewriters and its sister company Triumph-Adler were sold in 1986 to Olivetti and is since September 2004 a private American company again and today known as Royal Consumer Information Products Inc.

The featured Royal Model 91K calculator was manufactured by Omron in Japan and is a member of a design series that comes in three different sizes:

Dismantling the featured Royal Model 91K calculator manufactured in September 1975 by Omron 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 a µPD947 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 µPD947 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 µPD947 located in this Royal Model 91K and the µPD946 used with the Royal RSC-40, 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 µPD947 located in the featured calculator is a variation of the µPD946, an enhanced version of the µPD940 - one of the first "true" single-chip calculator circuits designed by NEC. The µPD946 features like the µPD940 an integrated clock oscillator and both its segment and digit output drivers are interfacing directly with low-voltage VFDs up to 30 Volts. Here at the Datamath Calculator Museum we don't qualify NEC's earlier µPD271 as a true single-chip calculator circuit, it is using with the µPD261 an external segment decoder and driver chip for the calculator display.

Display: The featured Royal Model 91K calculator manufactured in September 1975 makes use of an 9-Digit low-voltage VFD manufactured by Futaba and known as Type 9-ST-12, soldered with its 19 pins 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. NEC on the other hand entered the marked of single-chip calculator circuits in 1973/1974 and focused immediately on compatibility with VFDs. The µPD947 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 Royal Model 91K calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (150k Ohm) to around -27 Volts, the filament is biased to around -23 Volts (Zener Diode) and the µPD947 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Royal Model 91K makes use of the internal clock oscillator of the µPD946 Series of single-chip calculator circuits, we identified a capacitor with 1,500 pF connected between Pin 28 (CLK/C_EXT) of the µPD947 and the positive V_SS power supply line, resulting in a clock frequency of about 65 kHz.

Power Supply: The Royal Model 91K calculator is powered with four disposable AAA-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 Royal Model 91K calculator for two different cases:

Calculating the power consumption at 6 Volts for the Royal Model 91K results in about 120 mW displaying a '0.' and about 220 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 Royal Model 91K with the Date code 50.8.28 was manufactured by GICO in August 1975 and uses 3 sliding switches and 24 spring-supported plastic keys pushing small fingers on stamped sheet-metal pieces against contacts etched on a single-sided phenolic PCB.

While most single-chip calculator circuits are using their digit driver outputs to scan the keyboard matrix, decided NEC to utilize with the µPD946 Series the so-called segment scanning technology. The first part of a complete scanning cycle outputs the corresponding display information for the nine digits on the segment outputs, and the second part blanks the display and scans the segment outputs A to H and DP for possible keyboard actions. A 10^th keyboard row is connected directly to the V_SS power supply line to accommodate keyboards with up to 40 keys, greatly improving the limit of 24 contacts of the µPD940 Series. The layout of the keyboard assembly of the featured Royal Model 91K calculator shows consequently an arrangement with 10 keyboard scan lines and 4 keyboard return lines.

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.

While the keyboard of the DCM-50A Platform natively supports the segment scanning approach utilized with the NEC µPD946 Series, is it limited to the segment outputs A to G and DP as known from Texas Instruments' TMS0950 and TMS0970/TMC0900 Product Families. To overcome some of the limitations of the DCM-50A keyboard, we developed here at the Datamath Calculator Museum the DCM50A Playground KBD123 Keyboard with Switch Matrix. It is plugged on top of the DCM-50A Platform and centered around a 12x3 switch matrix keyboard with patch field for selector switches with diode matrix. All pins of the matrix (12 Columns, 3 keyboard rows and 1 switch row) are directly accessible on pin headers and can be connected with the matching pins on the DCM50A Playground DIL42 or DCM50A Playground BB400 Daughter Boards.

Comparing the Calculator Logic Implementation of the µPD947 retrieved from the featured Royal Model 91K with the Calculator Logic Implementation of an original µPD946 chip reveals four major differences:

Royal offered with its Model 90K (UA120) a calculator with a very basic feature set in the same housing as this Model 91K (UA122) and powered by the NEC µPD943 single-chip calculator circuit.

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

© Joerg Woerner, January 25, 2025. No reprints without written permission.