Texas Instruments TI-150

Texas Instruments introduced with this TI-150 in December 1974 a rather unusual calculator combining the wedge-shaped housing of the TI-2550 with the basic functionality of the Datamath II a.k.a. TI-2500-II and a Panaplex Style gas-discharge display. A small detail reveals the position of the TI-150 in Texas Instruments' "Basic Calculator Portfolio": The silver trim around the display frame, known from the first generation of the SR-10, placed it obviously in the upper-right corner of the price over performance chart.

The Date code 465 stamped on the backside of the featured TI-150 would indicate a manufacturing date in week 46 of November in the year 1975, to our understanding rather unlikely. We assume that this calculator was serviced in November 1975.

Dismantling this TI-150 manufactured in November 1974 reveals a design centered around a TMS0852 single-chip calculator circuit compared to e.g. the TMS0803 located in the TI-2500-II. Please notice the Date code 7443 (third week of October 1974) printed on the TMS0852.

The TMS0852 found its way with slightly modifications into the Canon Palmtronic LD-series. Don't miss the LD-80.

The TMS0852 chip is complemented on the main printed circuit board (PCB) by discrete transistors to drive the high-voltage gas-discharge display and a power supply.

The keyboard PCB of the TI-150 is identical to the TI-2550 but lacks the upper row of keys for the Memory functions.

Update (September 2024): On our quest to Record the ROM Content of the TMS0852 single-chip calculator circuit, we salvaged one of our TI-150 calculators (Yes, they are rare...) and studied its electronic circuitry completely. Disassembling the donor calculator with serial number #16294 and manufactured in November 1974 in Texas, resulted in three surprises:

Calculating Unit: The TMS0852 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 Light Emitting Diode (LED) displays and a clock generator. The TMS0850 Product Family uses redesigned segment and digit output drivers, directly interfacing with low-voltage Vacuum Fluorescent Displays (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 TMS0850 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 printed circuit board (PCB) traces of the donor TI-150, you'll recognize that Pin 10 (WDK) and Pin 8 (KN) are connected to effectively disable the Timeout feature. Desoldering the TMS0852 from the PCB and operating it with the Timeout feature enabled, results in a big surprise! The display stayed on, even after 5 minutes without any key presses.

Display: The TI-150 donor calculator manufactured in November 1974 makes use of a 9-Digit Panaplex Style 7-Segment display assembly manufactured by Texas Instruments. Here at the Datamath Calculator Museum we refer to this display as "DIS-PS9" till we learn about its actual part number. Burroughs developed the original Panaplex technology around 1970 as an alternative to Nixie or cold cathode displays, but the breakthrough came with the introduction of the second generation products, commonly known as Panaplex II numerical displays. Panaplex Style displays are using a hermetically tight sandwich construction filled with neon gas with thin and nearly transparent anodes for each digit below the top glass. The cathodes are printed in thickfilm technology with a conductive material direct on the ceramics substrate. If a voltage of typically around 180 Volts is applied between a segment and its cathode, the neon gas begins to glow in a warm orange. A mounting frames holds the Panaplex Style display and secures the 19 clamped pins connecting it to the Main-PCB. The injection molded plastic frame of the display module from the featured TI-150 calculator misses the typical stamped drawing number and bears instead a hand-scribed "TI 1030983" document number.

Display Driver: Multiplexed Panaplex Style displays are using a common anode layout for its digits with all segments (cathodes) bussed together and fire at a voltage much higher than 35 Volts, the maximum voltage allowed at the outputs of the TMS0850 single-chip calculators circuit. The PCB of the TI-150 calculator consequently uses a large amount of real estate not only for high-voltage generation but adequate display drivers, too. Texas Instruments chose to connect the digit driver outputs of the TMS0852 chip directly to the anodes of the display while its segment outputs use level shifters to a voltage of almost -200 Volts. Analyzing the circuitry of these level shifters consisting of one PNP bipolar junction transistor (BJT), three resistors and one high-voltage capacitor per segment channel, reveals a typical approach with a "voltage doubler". With these designs a capacitor is pre-charged to around -100 V and to activate the segment, the capacitor is connected to a bias voltage of -100 V, resulting - while the electron storage lasts in the capacitor - to a voltage of -200 V, enough to fire the corresponding segment. All other digits are not selected by the TMS0852 digit driver outputs and hence stay dim.

Clock: The TI-150 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 TMS0852 and the negative power supply line, resulting in a clock frequency of about of 160 kHz.

Power Supply: The TI-150 calculator is powered with four 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 dismantled TI-150 calculator:

Calculating the power consumption at 6 Volts for the TI-150 results in about 300 mW displaying a '0.' and about 340 mW with all segments but the minus sign illuminated. 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 of the TI-150 uses the proved Klixon™ hermetic miniature and sub-miniature snap action switches developed already in 1960. The Keyboard-PCB is connected with 13 stamped metal connectors to the Main-PCB.

With the DCM-50A Platform developed to Characterize and Reverse-engineer Single-chip Calculator Circuits we could proof that the TMS0855 uses very similar Program Code to the TMS0803 known from the TI-1500 or TI-2550-II but removed the Timeout feature completely.

How rare is the TI-150?

This is for calculator collectors the most important question - and the TI-150 seems to be pretty rare. Texas Instruments spent a fortune to develop this oddball in its calculator line (unique TMS0852 single-chip calculator circuit, miniaturized Panaplex Style gas-discharge display, unique AC9150 AC-Adapter) but left the design of the sales packaging unfinished.

Roger Whitaker, Lead Engineer of the TI-150 confirmed an estimated number of about 4,000 produced units:

The TI-150 was the only handheld to use a plasma display. It has a character height of 0.2 inch and was intended as an "Executive" calculator. The case was the TI-2550's. The comments on Joerg's page are basically correct. It lacked the memory feature and was over priced. The production figure of 4,000 is probably right, although I wonder if they actually sold that many. It definitely was the wrong calculator at the wrong time. The plasma display requires about 150 volts to fire the segments. The TMS0855 is probably similar to the TMS0852 in having relatively high voltage display drive output architecture, but driving the plasma display required use of separate discrete high voltage PNP transistors, 2N5400 or 2N5401's as I recall without opening up my TI-150 to look. My 150 is Serial Number 010038 which was a prototype. Something else that most people may not know is that TI made its own plasma displays at a plant in Sherman. They were a direct replacement for the Burroughs Panaplex. I think that my TI-150 may have one of them. They also made the larger ones for the desktop units, too.

And in case you think the TI-150 is rare - how about a TI-150 Prototype?

Fellow collectors - if you own a Texas Instruments TI-150 calculator,  please report us the serial number and date code from the back of the calculator (optionally from the TMS0852 calculator chip) for our Database.

Datamath™ is a trademark of Texas Instruments.

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

© Joerg Woerner, December 5, 2001 and March 14, 2020. No reprints without written permission.