Canon Palmtronic LE-84 (Display Version 3)

When Canon introduced in January 1974 with the LE-83 the first member of their 4^th Generation of Battery-powered Handheld Calculators with LED display, they continued their successful cooperation with Texas Instrument and decided for the TMS0101, known already from the LE-80 introduced in 1972. But not that TMS0101, the other one....

Texas Instruments introduced in Fall 1973 the successor of the 1^st Generation TMS0100 Product Family, and they diversified the portfolio into three different branches:

The LE-84 introduced a few months later in May 1974 uses the TMS0801 calculator chip instead of the TMS0101, certainly a viable approach to reduce manufacturing costs while adding features without sacrificing the build quality of a battery operated calculator. We assume that the TMS0801 was the first application of the TMS0800 Product Family, from the technology this LE-84 is similar to the TI-1500 introduced by Texas Instruments just one month earlier.

Dismantling the featured Canon LE-84 calculator manufactured in November 1974 in Japan requires the removal of two screws located under the battery compartment and is instantly rewarding with the very high manufacturing quality of Canon products. Every part of the calculator feels very substantial and well engineered, from the sliding battery cover to the keyboard assembly.

We started on our quest to Record the ROM Content of the TMS0801 single-chip calculator circuit deeper into the internals of various LE-84 calculators and made some interesting the findings:

Calculating Unit: The TMS0801 is a member of 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.

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 featured Canon LE-84, you'll recognize that Pin 10 (WDK) and Pin 8 (KN) are connected to effectively disable the Timeout feature.

Display: The featured Canon LE-84 calculator manufactured in November 1974 makes use of ROHM LAB-B-2 8-Digit display module with nine 7-Segment displays chips bonded onto a PCB and magnified with a clear plastic lens. The display module is connected with 16 pins to the Main-PCB and follows the industry standard pinout of 9-Digit modules with the leftmost pin (Digit D9) unconnected. A small red LED with long leads is soldered directly onto the Main-PCB of the LE-84 calculator and bended over the display module to act as Minus "dot" for 8-digit numbers.

With more than a dozen of LE-83, LE-84 and LE-85 calculators examined, we understood that Canon used four different LED display manufacturers and technologies with their 4^th Generation of Battery-powered Handheld Calculators and even marked the backside of the calculators accordingly. The featured calculator as an example has a round paper sticker with the number '54' attached to its backside, the secret code for an ROHM LAB-B-2 display. Not all display technologies were used with each calculator, as of today we identified the following matrix:

Note: Here at the Datamath Calculator Museum we use a "Version Index" for the different display modules located in the Canon LE-83, LE-84 and LE-85 calculators based on the round paper stickers but couldn't find out the chronological order of them.

Display Driver: The Main-PCB of the featured Canon LE-84 (Version 3) manufactured in November 1974 makes use of nine discrete 2SC1641 NPN bipolar junction transistors (BJTs), while our featured LE-84 (Version 2) manufactured in April 1974 makes makes use of two hybrid modules with five, respective four transistors, each. Looking closer at the layout of the PCB, you'll notice that there are "spare" holes to the right of the transistors to accommodate the hybrid modules, too. Understanding the circuit diagram of the almost identical Canon LE-85, we felt safe to measure the Output Voltage vs Input Voltage transfer function of its hybrid modules with an automated setup originally created for 75492-style devices. Here at the Datamath Calculator Museum we use this transfer function as an easy obtainable "Signature" of the underlying circuit design and manufacturing processes of digit drivers. In a first step we tested one transistor of the hybrid modules each with a DCA75 Advanced Semiconductor Analyzer from Peak Electronic Design. The DCA75 easily recognized the transistors as NPN BJT and measured a current gain (h_FE) of around 160 for the first transistor of the 5-channel hybrid module and around 260 for the first transistor of the 4-channel hybrid module.

Next we retrieved the Signatures of the tested transistors and compared it with a discrete 2N3904 NPN BJT with a current gain of around 180. Comparing the three Signatures demonstrates clearly that the two hybrid modules contain just discrete General Purpose NPN BJTs bonded on a ceramic substrate and no other components like resistors or diodes.

Surprised by the rather high current consumption of the featured Canon LE-84 (Version 3) calculator, we measured the current gain (h_FE) of one of its 2SC1641 NPN BJTs and found it around 510. Retrieving the Signature of the 2SC1641 results consequently in a very steep Output Voltage vs Input Voltage transfer function and hence high segment and digit currents of the LED display.

Clock: The featured Canon LE-84 makes use of the internal clock oscillator of the TMS0800 chip, we identified a resistor with 100 kOhm connected between Pin 14 (R_EXT//Clock Select) of the TMS0801 and the V_DD power supply line, resulting in a clock frequency of about of 165 kHz.

Power Supply: The Canon LE-84 is powered by four disposable AA-sized 1.5 Volt batteries and can be operated with an external, DC adapter, too. The PCB of the calculator hosts a discrete power converter to generate the V_DD and V_GG supply voltages for the TMS0807 chip. We observed with the featured calculator manufactured in November 1974 voltages of V_DD = -9.2 V and V_GG = -16.3 V while operated with V_BAT = 6.0 V. While reverse-engineering the circuit diagram of the calculator, we noticed the rather low 2k2 Ohm series resistors for the digit drivers, usually a sign of high segment currents of the LED display. The PCB of the featured calculator did confirmed our suspicion, the populated segment resistors are with 220 Ohm much lower dimensioned than with both a previously dismantled LE-84 (Version 1) and LE-84 (Version 2) manufactured in June and April 1974, respectively and using only 220 Ohm. Consequently did we measure a very high operating current of the featured LE-84 calculator:

The power consumption for the featured Canon LE-84 results in about 130 mW displaying a '0.' and 490 mW with all segments but the minus sign illuminated, barely within the 500 mW stated on the label of the calculator. This compares to around 100 mW and 310 to 320 mW of the two other tested LE-84 display variations. To fully understand the power budget of the Canon LE-84, we not only calculated the theoretical segment and digit currents of the featured calculator, we measured them dynamically with the calculator operating.

With the operating current numbers from the featured Canon LE-84, we calculate in a first step the average current per segment. The differences between the '0.' and '88888888.' displays are 50 segments and 60 mA, or 1.2 mA per segment. The TMS0801 chip is scanning the digits and segments with a 1:10 duty cycle, resulting in 12.0 mA average segment current per Digit Time. Each Digit Time has 11 State Times with S1 and S11 blanking the segments for a peak current of 14.7 mA per segment and around 103 mA per digit. The high-efficiency LED chips used with the calculator have a voltage drop of around 1.7 V at 14.7 mA segment current, and with a 6 Volt battery the sum of all other components in the loop will "burn" around 4.3 V:

The true master is Texas Instruments with its TI-2500-II design, using only two 1.5 Volt batteries and "burning" just 1.3 V between power supply and LED chips.

Keyboard: The Canon LE-84 calculator makes use of a keyboard module with a single-sided phenolic-substrate PCB with gold-plated traces for the contacts and spring-loaded conductive rubber elements for its injection molded plastic keys. This technology used by Canon with many calculators form the early 1970s proved over time as very reliable.

With the DCM-50A Platform developed to Characterize and Reverse-engineer Single-chip Calculator Circuits we could proof that the Program Code of the TMS0801 matches the example described in Texas Instruments' Patent Application US3934233A covering the TMS0800 architecture.

The LE-84 and its sibling LE-85 were the last two pocket calculators developed by Canon and using red LED (Light Emitting Diode) displays.

Here at the Datamath Calculator Museum we classify the featured LE-84 as Display Version 2.

Canon's next calculator generation starting with the Panther D and LD-80 used VFDs (Vacuum Fluorescent Displays). Main advantage of this technology was that time both lower power consumption and cheaper manufacturing costs.

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

© Joerg Woerner, September 12, 2024. No reprints without written permission.