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Interton PC 2008 (Version 1)

Date of introduction:	January 1973	Display technology:	LED-stick
New price:		Display size:	8
Size:	5.2" x 3.0" x 1.05"  133 x 76 x 27 mm3
Weight:	5.3 ounces, 150 grams	Serial No:	110017
Batteries:	4*AA	Date of manufacture:	mth 01 year 1973
AC-Adapter:		Origin of manufacture:	Germany
Precision:	8	Integrated circuits:	General Instrument TMR 012, 2*SN75491, RCA CA3082
Logic:	Chain	Displays:	Bowmar Optostic R7H-112-9-style
Memories:
Program steps:		Courtesy of:	Joerg Woerner
		Download manual:	(DE: 1.0M Bytes)

Interton Electronic Hörgeräte was founded in 1962 by the brothers Hellmuth Jakob Türk and Hans-Herbert Türk in Cologne as a manufacturer of hearing aids. In January 1973, Interton introduced with the PC 2008 its first electronic calculator before adding in 1975 video game consoles to its portfolio.

We acquired this Interton PC 2008 (Version 1) calculator in 2025 on our quest to complete the Characterization of Single-Chip Calculator Circuits of General Instrument's C-500 Product Family of Single-chip Calculator Circuits - just to find out that its brain is marked with TMR 012 instead the expected C-550.

Dismantling the featured Interton PC 2008 (Version 1) calculator manufactured in January 1973 in Germany reveals a rather complex design based on two double-sided printed circuit boards (PCBs) for its electronics and keyboard, a display module and powered by four AA-sized batteries or an external power adapter.

The Main-PCB is centered around a General Instrument TMR 012 single-chip calculator circuit and supported by eighteen resistors for its keyboard interface and two additional transistors with some resistors, capacitors and an inductor generating the different supply voltages for the TMR 012 and its clock signal. A CA3082 transistor array and one discrete transistor are used as segment drivers for the LED display module provided by Bowmar. "Demultiplexing" the digit output signals of the TMR 012 and driving the common cathodes of the LED display is realized with two transistors and two SN75491 drivers.

To gain some knowledge about the differences between the TMR 012 located in this PC 2008 (Version 1) and the C-550 used with the PC 2008 (Version 2), we decided here at the Datamath Calculator Museum to give it a "Teardown Treatment" and sharing our findings accordingly.

Calculating Unit: The TMR 012 used with the Interton PC 2008 (Version 1) is a member of the second generation of General Instrument single-chip calculator circuits and traces back to the famous PICO1 introduced already in 1971:

Display: The featured Interton PC 2008 (Version 1) calculator manufactured in January 1973 makes use of a Bowmar Optostic module similar to the type R7H-112-9. It uses seven individual GaAsP (Gallium Arsenide Phosphide) Segment LED chips and one GaAsP Decimal Point LED chip per character bonded directly to a double-sided FR4 printed circuit board (PCB) and placing a one-piece red acrylic protecting lens on top of the assembly with four heat stakes. The leftmost display position is missing like the R7H-112-9 the upper right segment, but sports the decimal point. The display module is connected with 17 pins to the Main-PCB of the calculator, but the left most pin (Digit D9 Common Cathode) is not used.

Display Driver: Early single-chip calculator circuits like the PICO1, Mostek's MK6010 or Texas Instruments' TMS0100 didn't include any display drivers and left the choice of display technology and necessary interface circuitry to the designers of the electronic calculators. The PMOS (p-channel Metal–oxide Semiconductor) technology used with these chips was neither compatible with Vacuum Fluorescent Displays (VFDs) nor with Led Emitting Diode (LED) Displays:

Progress with both display technology and chip technology allowed around 1972/1973 the direct connection of low-voltage (< 35 Volts) VF Displays to single-chip calculator circuits with modified "high-side" segment and digit output drivers to withstand up to -35 Volts. In a next step, around 1973, the used PMOS chip technology allowed beefier "high-side" segment drivers while the LED displays exhibited increased efficiency, meaning they could be operated with lower currents. The digit drivers still used external "low-side" drivers. Rockwell introduced in June 1974 with the A5300 Product Family the first single-chip calculator circuits with LED (Light-Emitting-Diode) Direct-Drive capability, integrating not only the segment drivers but the digit drivers, too.

The General Instrument TMR 012 chip uses only 4 digit outputs, labeled D15, D26, D37, and D48, requiring an external "demultiplexer" to operate an 8-digit display. The dismantled Interton PC 2008 (Version 1) calculator uses a very simple and straightforward approach to interface with the LED display:

Clock: The TMR 012 single-chip calculator circuit of the Interton PC 2008 (Version 1) is operated with a frequency of about 75 kHz.

Power Supply: The Interton PC 2008 (Version 1) calculator is powered with four disposable AA-sized alkaline batteries or an external 6 Volt power adapter and uses a simple DC/DC converter to generate an additional negative voltage for the TMR 012:

We measured the operating current of the featured Interton PC 2008 (Version 1) calculator for two different cases:

Calculating the power consumption at 6 Volts for the Interton PC 2008 (Version 1) results in about 1,220 mW displaying an idle '00000000' and about 1,260 mW with all segments illuminated. An unusual high value and clear indicator of the missing "trailing zero suppression" of the early General Instrument single-chip calculator circuits.

Keyboard: The keyboard of the Interton PC 2008 (Version 1) uses 19 snap action switches mounted directly on a second PCB of the calculator together with a sliding switch for power. Please notice that the [C] and [CE] keys of the Interton PC 2008 calculator are hardwired on the Keyboard-PCB together and act as a single [C] key.

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.

On our quest to document Pico Electronics' PICO1 Chip and its many descendants like the General Instrument C-500, C-550, CZ-550, C-560, C-570, CZL-550, and this  TMR 012, we developed here at the Datamath Calculator Museum three additional tools for our DCM-50A (PLAYGROUND):

Comparing the Calculator Logic Implementation of the TMR 012 retrieved from the featured Interton PC 2008 (Version 1) calculator with the Calculator Logic Implementation of the C-550 chip used with the Interton PC 2008 (Version 2) reveals no differences and we decided to go down the rabbit hole of understanding the differences between the two chips used in the same calculator model within a timespan of just three months.

Comparing the PCBs of the two chips reveals no differences other than the values of the segment resistors (39 Ohms and 43 Ohms), neither does the current consumption of the calculator chips.

Comparing the Segment Driver Output Characteristics of the TMR 012 with the Segment Driver Output Characteristics of the C-550 reveals no major differences, either.

In our final attempt, we decided to "decap" both the TMR 012 and C-550 chips salvaged from two Interton PC 2008 calculators and asked Sean Riddle to provide us with high-resolution images of the silicon die.

First surprise: The TMR 012 chip looks very familiar to us, it is identical to the C-500 chip, the 25 Volt predecessor of the 15 Volt C-550. Looking closely on the die of the C-500 and TMR 012 chips reveals Mask Revision G, all five Masks have a small 'G' assigned.

Second surprise: The C-550 chips uses with four Masks the same Revision G but is suggesting for Mask 5 (Pad Opening Mask) Revision L, most likely a hint to the "low-voltage" specification of the C-550. We couldn't find any differences in the Pad Openings of the original C-500 chips and the C-550 chips and from a technical point of view, changes of Mask 5 wouldn't change the operating specifications of the design. We rather assume that the character "G" could be easily changed to "L" on the rubylith.

We assume that General Instrument was using a 2-step approach while transitioning the process parameters from the original 25 Volt C-500 design to the later 15 Volt specifications of the C-550 and selling the interims chips as TMR 012 to Interton. Based on the serial numbers observed with the Interton PC 2008, we assume that less than 10,000 TMR 012 chips were manufactured before the C-550 was introduced as standard product.

Don't miss the "Picolator" on the German Richi's Lab site, an Emulator for General Instrument's C-550 single-chip calculator circuit - still using the PICO1 program code from the original GI 250 chip.

The Interton PC 2008 calculator was upgraded later in 1973, featuring both [%] and [‰] keys and adding two additional sliding switches for Constant and and Fixed-DP or Floating-DP operation. The calculator brain of the PC 2008 (Version 3)? A TMS0128 from Texas Instrument.

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

© Joerg Woerner, November 13, 2025. No reprints without written permission.