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Brother PROCAL 408AY (Version 2)

Brother Industries, Ltd. was founded already in 1908 as Yasui Sewing Machine Co, in Nagoya, Japan and is today mainly known for their printers and sewing machines and vaguely remembered for their typewriters. Nevertheless entered Brother already in October 1966 with the fully-transistorized Calther 130 the business of electronic calculators before switching in October 1969 with the Calther 412 (12 Nixie tubes) and Calther 514 (14 Nixie tubes) to a mix of Integrated Circuits (ICs) and transistors. Shortly after Texas Instruments introduced in October 1971 the TMS1802NC "calculator-on-a-chip", Brother introduced with the PRO-CAL 408 one of the earliest battery powered, portable electronic calculators.

The original PRO-CAL 408 calculator went through different cost-reduction programs and as of today we identified four different versions:

A next mayor step was the introduction of this PRO-CAL 408AY utilizing the first NEC single-chip calculator circuit:

Here at the Datamath Calculator Museum we acquired in 2025 a Brother PROCAL 408AY (Version 1) calculator and this PROCAL 408AY (Version 2) on our quest to complete the Characterization of Single-Chip Calculator Circuits manufactured by NEC (Nippon Electric Company) Corporation of Japan.

Dismantling the featured PROCAL 408AY calculator manufactured in January 1974 in Japan reveals a cost effective design using on a single-sided printed circuit board (PCB) for the main electronics and powered by four disposable 1.5 Volts batteries or a 6-Volt DC power adapter.

The Main-PCB is centered around a µPD273 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 µPD273 and Vacuum Fluorescent Display (VFD) and to bias the anodes and grids of the display with respect to its filament.

To better understand the functionality of the rather uncommon µPD273 located so far only in the PROCAL 408AY, 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 µPD273 located in the featured calculator is considered NEC's first single-chip calculator circuit with 8-digit display capability and very basic functionality, following the µPD271 and its low-voltage sibling µPD272. Here at the Datamath Calculator Museum we don't qualify the 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. The µPD273 added both an internal clock oscillator and segment decoder to the feature set of its predecessors, rendering it a "true" single-chip calculator circuit. It was complimented in December 1973 with the µPD277, a design with 8-digit display capability and integrated 2-key Memory.

Display: The featured Brother PROCAL 408AY calculator manufactured in January 1974 makes use use of a 9-Digit DP90A low-voltage VFD manufactured by Ise Electronics, inventor of the Vacuum Fluorescent Display and today better known as NORITAKE ITRON CORPORATION. The DP90A is soldered on s small PCB which is connected with 19 short wires to the Main-PCB of the calculator.

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 µPD273 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 Brother PROCAL 408AY calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (50k Ohm) to around -25 Volts, the filament is biased to around -23 Volts (Zener Diode) and the µPD273 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Brother PROCAL 408AY makes use of the internal clock oscillator of the µPD273 single-chip calculator circuit with two dedicated pins and four external components. The primary section of the internal clock oscillator, Pin 1 (CR1/R_EXT/C_EXT) requires a resistor R_EXT1 to V_GG and a capacitor C_EXT1 to V_SS. The secondary secondary section, Pin 28 (SR2/R_EXT/C_EXT) requires another resistor R_EXT2 to V_GG and another capacitor C_EXT2 to V_SS. We identified two resistor/capacitor combinations R_EXT1/C_EXT1 with nominal values of 150 kOhm and 100 pF, respectively and R_EXT2/C_EXT2 with nominal values of 300 kOhm and 100 pF for a typical frequency of 36 kHz.

Power Supply: The Brother PROCAL 408AY 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 Brother PRO-CAL 408AY calculator for two different cases:

Calculating the power consumption at 6 Volts for the Brother PROCAL 408AY results in about 230 mW displaying a '0.' and about 290 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 Brother PROCAL 408AY (Version 2) uses 19 plastic keys pushing small conductive carbon discs mounted in two large silicone rubber membranes against a thin, self-adhesive plastic foil with openings for contacts etched on the Main-PCB and using a highly conductive plating. The sliding switch for Power On/Off and the Constant function of the calculator is wired to the Main-PCB. A closer look at the keyboard contacts shows the center contacts connected to one of the ten digit-driver outputs of the µPD273 and their outer perimeter connected with the key-matrix inputs NK and FK of the µPD273. The [C] key is connected directly between common voltage V_SS and the CL input of the µPD273. Don't miss the unusual "capacitive" keyboard using conductive carbon discs used with the PROCAL 408AY (Version 1).

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.

Investigating the Calculator Logic Implementation of the µPD273 retrieved from the Brother PROCAL 408AY reveals some surprises.

The Constant Function uses a very unusual - and buggy - approach for Multiplication (1^st number used as constant), Division (2^nd), Addition (1^st), and Subtraction (1^st) that we refer here in the Datamath Calculator Museum as (M-D-A-S) 1-2-1-1 implementation. The µPD277 dropped Addition and Subtraction from the Constant Function for an (M-D-A-S) 1-2-X-X implementation, while the later µPD276 expanded it again to a more common (M-D-A-S) 1-2-2-2 implementation. The µPD273 was plagued with another bug related to the Percent Function, is displaying after some calculations a negative zero and entering a ninth digit is resulting in an inconvenient overflow condition.

Consequently had the µPD273 a very short lifecycle and was replaced soon with the µPD940, adding more functionality and other improvements to reduce manufacturing costs of battery-operated handheld calculators.

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

© Joerg Woerner, April 18, 2025. No reprints without written permission.