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MBO de Luxe IV

The MBO International Electronic GmbH, Jena was founded 1973 and started with the MBO Junior the production of the first portable calculator in Germany. In the following years, MBO placed their brand label on various products manufactured by Original Equipment Manufacturers (OEMs), this MBO de Luxe IV calculator was manufactured by Toho Tsusho Co., Ltd. and is also known as Tohotronic Roger Fighter F-5.

Dismantling the featured MBO de Luxe IV calculator manufactured in December 1975 in Japan reveals an unusual design based on a single-sided printed circuit board (PCB) for the main electronics separated by a wide flat cable from a single-sided display PCB to bridge the two center-located AA-size batteries. The keyboard assembly is connected with a second, shorter flat cable to the Main-PCB.

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 µPD941 located in the MBO de Luxe I, the µPD940 used with the MBO de Luxe II, the µPD277 used with the MBO de Luxe III (Version 1), the µPD278 used with the MBO de Luxe III (Version 2) and this µPD946, we decided here at the Datamath Calculator Museum to give the featured calculator a full "Teardown Treatment" and share our findings accordingly.

Calculating Unit: 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 like the MBO de Luxe III (Version 1). 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.

For calculator manufacturers, a product portfolio with such a small granularity was a perfect way to offer the right function set for the right price to their customers:

Display: The featured MBO de Luxe IV calculator manufactured in December 1975 makes use of an 9-Digit low-voltage VFD manufactured by Itron and known as Type DP95A4, soldered with its 19 wires to a small Display-PCB which is connected with a flat cable to the Main-PCB.

Display Driver: The term "low-voltage" Vacuum Fluorescent Display might be misleading when used together with a calculator powered by two 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 MBO de Luxe IV calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (100k Ohm) to around -28 Volts, the filament is biased to around -26 Volts (Zener Diode) and the µPD946 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The MBO de Luxe IV makes use of the internal clock oscillator of the µPD946 single-chip calculator circuit, we identified a capacitor with 1,500 pF connected between Pin 28 (CLK/C_EXT) of the µPD946 and the V_SS power supply line, resulting in a clock frequency of about 55 kHz.

Power Supply: The MBO de Luxe IV calculator is powered with two disposable AA-sized 1.5 Volt batteries or an external 3 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 MBO de Luxe IV calculator for three different cases:

Calculating the power consumption at 3 Volts for the MBO de Luxe IV results in about 180 mW displaying a '0.' and about 240 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 MBO de Luxe IV uses 22 snap action switches and two sliding switches for power and the Memory-Accumulate function soldered on a single-sided phenolic PCB. The keyboard module is connected with 15 wires to the Main-PCB and batteries.

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 31 contacts of the µPD277. The layout of the keyboard assembly of the featured MBO de Luxe IV 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.

Comparing the Calculator Logic Implementation of the µPD946 harvested from the featured MBO de Luxe IV with the Calculator Logic Implementation of the µPD278 chip from an MBO de Luxe III (Version 2) reveals some interesting 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, April 5, 2025. No reprints without written permission.