Privileg 804 D

Quelle AG was - together with its arch rival Neckermann - one of the leading department stores in Germany and distributed most of their electronic products under the "privileg" label. This Privileg 804 D calculator caught our attention in 2024 with its unusual [D] key, in most cases a hint about relying on a Texas Instruments single-chip calculator circuit from the TMS0800 Product Family.

Dismantling the featured Privileg 804 D calculator manufactured in May 1975 by an unknown OEM (Original Equipment Manufacturer) in Far East reveals a clean design based on a single-sided printed circuit board (PCB) for the main electronics, a double-sided PCB for the keyboard and powered by two disposable 1.5 Volts batteries.

The Main-PCB is centered around a TMS0851 single-chip calculator circuit manufactured by Texas Instruments and. The few remaining components on the PCB are mainly used to generate the different supply voltages for the TMS0851 and Vacuum Fluorescent Display (VFD) and to bias the anodes and grids of the display with respect to its filament..

On our quest to Record the ROM Content of the TMS0851 single-chip calculator circuit, we decided here at the Datamath Calculator Museum to give the featured Privileg 804 D calculator a full "Teardown Treatment" and share our findings accordingly.

Calculating Unit: The TMS0851 is closely related to the TMS0800 Product Family and tracing back to the TMS1802NC, the first available standard calculator building block on a chip, later renamed into TMS0102. The TMS0800 kept the size of the Instruction ROM (Read-Only Memory), but decreased the Data Memory from 13 Digits Registers to 11 Digit Registers and added both integrated Segment Drivers for the LED display and a clock generator. The TMS0850 Product Family uses redesigned segment and digit output drivers, directly interfacing with low-voltage VFDs up to 35 Volts and features modified keyboard scanning inputs that can withstand up to 35 Volts.

With low-cost battery operated LED calculators in mind, Texas Instruments added a so-called Timeout feature to the TMS0800 devices. When no key presses are detected for about 20 seconds, the display blanks out and shows only a '-' in the leftmost digit to reduce power consumption of the calculator. Looking closely at the PCB traces of the dismantled Privileg 804 D, you'll recognize that Pin 10 (WDK) is connected to the keyboard to "recall" the calculator with its [D] key. Does it make a difference with a battery operated VFD calculator? Not really.

Display: The Privileg 804 D donor calculator manufactured in May 1975 makes use of an 8-Digit DP89A low-voltage VFD manufactured by Ise Electronics, inventor of the Vacuum Fluorescent Display and today better known as NORITAKE ITRON CORPORATION. The display is soldered with its 28 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 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 1975 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. The TMS0850 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 the Privileg 804 D calculator, too. The grids and anodes of the VFD are "pulled-down" with 16 resistors (180k Ohm) to around -32 Volts, the filament is biased to around -30 Volts (Zener diode) and the TMS0851 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Privileg 804 D makes use of the internal clock oscillator of the TMS0850 chip, we identified a resistor with 200k Ohm connected between Pin 14 (R_EXT/Clock Select) of the TMS0851 and the negative V_DD/V_GG power supply line, resulting in a clock frequency of about of 98 kHz.

Power Supply: The Privileg 804 D calculator is powered with two disposable AA-sized 1.5 Volt batteries and uses a simple DC/DC converter to generate a total of three voltages:

We measured the operating current of featured Privileg 804 D calculator for three different cases:

Calculating the power consumption at 3 Volts for the Privileg 804 D results in about 410 mW displaying a '0.', about 520 mW with all segments but the minus sign illuminated and around 230 mW in "battery saving" Timeout mode. Not very impressive, a Canon LE-84 calculator 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.

Keyboard: The keyboard assembly of the Privileg 804 D uses 19 snap action switches and a sliding power switch mounted on a double-sided printed circuit board (PCB). The keyboard module is connected with 13 pins and 2 wires to the Main-PCB and batteries.

With the DCM-50A Platform developed to Characterize and Reverse-engineer Single-chip Calculator Circuits we could proof that the TMS0851 uses the same Program Code as the TMS0803 known from the TI-1500 or TI-2550-II but disabled the Segment Blanking in State 1 and State 11 of the Digit Times.

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

© Joerg Woerner, September 14, 2024. No reprints without written permission.