Royal RSC-40

In 1965, Gordon E. Moore - co-founder of Intel - postulated that the number of transistors that can be packed into a given unit of space will double about every two years. This allowed to integrate more and more transistors, diodes and resistors into Integrated Circuits (ICs) and we differentiate between "Small Scale Integration" (SSI) with complexities in the lower tens of components, "Medium Scale Integration" (MSI) with tens to hundreds of components on a single chip and "Large Scale Integration" (LSI) with one thousand and more components integrated on a small silicon chip. The design of LSI ICs was mainly a result of switching from bipolar transistor technology to Metal-oxide Semiconductor (MOS) technology, allowing higher densities for transistors used with digital functions like logic gates and storage registers found in electronic calculators and computers. In 1969 it was commercially possible to integrate about 1,000 transistors in a p-Channel MOS (PMOS) process on a silicon die measuring about 0.2" x 0.2" (5 mm x 5 mm) and having about 28 to 48 electrical connections to the outside world while 1972 marked the year of having so-called single-chip calculator circuits available and paving the road to affordable, battery-operated handheld calculators.

Continuous improvements of chip manufacturing processes didn't stop and integrating more and more transistors on a silicon die, allowed to add extra functionalities to the single-chip calculator circuits:

By 1975 there were hundreds of companies manufacturing thousands of different models of pocket calculators all around the world and competition became very fierce, with market prices dropping dramatically. With every new battery-operated handheld calculator introduced mid of the 1970s, there was a simple choice behind:

The featured Royal RSC-40 is an "inbetweener" from the 1975-era, not your typical Basic calculator and not your typical Scientific calculator with some Convenience Functions shattered on the keyboard and accessible as 2^nd-functions with an additional [F] key.

Dismantling the featured Royal RSC-40 calculator manufactured in March 1976 by a unknown OEM in Taiwan reveals a very compact design based on a single-sided printed circuit board (PCB) for the main electronics, a double-sided PCB for the keyboard and powered by four disposable 1.5 Volts batteries.

The Main-PCB is centered around a µPD946 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 µPD946 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 µPD946 located in this Royal RSC-40 and the µPD947 used with the Royal's Model 91K, 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 µPD946 located in the featured calculator is 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 RSC-40 calculator manufactured in March 1976 makes use of an 9-Digit low-voltage VFD manufactured by Itron and known as Type DP95A, soldered with its 19 long 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. NEC on the other hand entered the marked of single-chip calculator circuits in 1973/1974 and focused immediately on compatibility with VFDs. The µPD946 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 RSC-40 calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (100k Ohm) to around -27 Volts, the filament is biased to around -23 Volts (Zener Diode) and the µPD946 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Royal RSC-40 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 µPD946 and the positive V_SS power supply line, resulting in a clock frequency of about 52 kHz.

Power Supply: The Royal RSC-40 calculator is powered with four disposable AA-sized 1.5 Volt batteries and uses a complex DC/DC converter to generate a total of four voltages:

We measured the operating current of the featured Royal RSC-40 calculator for two different cases:

Calculating the power consumption at 6 Volts for the Royal RSC-40 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 RSC-40 uses 24 plastic keys pushing small conductive carbon pills mounted in a large silicone rubber membrane against contacts etched on a double-sided phenolic PCB. The sliding switch for the Memory-Accumulate function of the calculator is directly wired to the Keyboard-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 30 keys and 10 switches, greatly improving the limit of 24 contacts of the µPD940 Series. The layout of the keyboard assembly of the featured Royal RSC-40 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 µPD946 retrieved from the featured Royal RSC-40 with the Calculator Logic Implementation of a µPD947 chip reveals four major differences:

While the layout of the keyboard matrix of the µPD946 Series allows for up to 30 keys and 10 switches, are typical "Basic" handheld-calculators using around 20 to 25 keys. NEC introduced with the firmware of the µPD946 and µPD947 an optional [F] key to access the Convenience Functions as 2^nd-functions to the [+], [×], [÷] and [=] keys and to access all implemented functionality, either a 28-key or a 24-key layout is possible.

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

© Joerg Woerner, March 4, 2025. No reprints without written permission.