MBO Expert (7C-903)

This Sovrin Model 128 calculator caught our attention 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. Sovrin, a European marketing firm, did not manufacture its own calculators, this Model 128 was assembled in Singapore by an unknown Original Equipment Manufacturer (OEM). Other Sovrin branded calculators came from OEM assembly lines in Japan, Malaysia and Taiwan. We know an almost identical product portfolio sold by the British company Plustronics Ltd. under the Plustron brand.

Dismantling this Sovrin Model 128 calculator manufactured in May 1975 in Singapore 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 other 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 Sovrin Model 128 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 Sovrin Model 128, 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 Sovrin 128 donor calculator manufactured in May 1975 makes use of a 9-Digit low-voltage VFD manufactured by Futaba and known as Type 9-ST-12. The display is 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 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. 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 Sovrin Model 128 calculator, too. The grids and anodes of the VFD are "pulled-down" with 17 resistors (150k Ohm) to around -32 Volts, the filament is biased to around -30 Volts (2.7k Ohm resistor) and the TMS0851 switches the relevant grids and anodes to around 0 Volt to lit them up.

Clock: The Sovrin Model 128 makes use of the internal clock oscillator of the TMS0850 chip, we identified a resistor with 150k Ohm connected between Pin 14 (R_EXT/Clock Select) of the TMS0851 and the negative V_DD power supply line, resulting in a clock frequency of about of 128 kHz.

Power Supply: The Sovrin Model 128 calculator is powered with two disposable AA-sized 1.5 Volt batteries and uses a simple DC/DC converter to generate a total of four voltages:

We measured the operating current of the featured Sovrin Model 128 calculator for three different cases:

Calculating the power consumption at 3 Volts for the Sovrin Model 128 results in about 300 mW displaying a '0.', about 340 mW with all segments but the minus sign illuminated and around 250 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 Sovrin Model 128 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 15 pins to the Main-PCB.

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 13, 2024. No reprints without written permission.