Canon LE-81M

The LE-81M seems to be the direct successor of the odd looking LE-81 calculator. It was introduced together with the LE-100 sporting 10-digits and the scientific calculator F-5. The LE-81M supports with a small sliding switch an "Adding Machine" mode using an independent add register that can be read out with a single press of the [T] key and cleared with a second consecutive press of the [T] key.

Dismantling the featured Canon LE-81M manufactured in June 1974 by Canon in Japan reveals a rather compact design with three printed circuit boards (PCBs) for main electronics, display, and keyboard powered by either a battery pack with four AA-sized rechargeable NiCd cells or a battery holder for four disposable Alkaline batteries. The Main-PCB is centered around a Texas Instruments single-chip calculator circuit and supported by a myriad of discrete components:

Calculating Unit: The LE-81M makes use of the TMC0605 single-chip calculator circuit derived from the TMS1802, better known as first "calculator-on-a-chip" but with a Read-Only program Memory increased from 320 Words to 384 Words x 11 Bits. The first design of the TMS0600 architecture could be found with the TI-2550 introduced already in January 1974 and centered around the TMS0601 chip.

Display: Texas Instruments introduced together with the TMS0100 calculator chip two pre-configured LED (Light-Emitting-Diode) modules (DIS40, DIS95) based on the TIL360 arrays and the corresponding segment drivers (SN75491) and digit drivers (SN75492). Most early 8-digit designs like the LE-80 made use of these parts exhibiting two disadvantages:

Texas Instruments consequently introduced with the SN75493 and SN75494 revised display drivers optimized for designs with 3 or 4 batteries but the true innovation could be observed with the technology how to manufacture the 9-digit LED displays used with early four-function calculators:

The featured LE-81M manufactured in June 1974 makes use of a Bowmar Optostic R7H-122-9 Nine-Digit display module using Chip-on-Board (COB) technology with each character formed by bonding seven individual Segment chips and one Comma chip bonded directly to a PCB and protected with a red plastic lens. The display module is connected with a long 17-pin flat-cable to the Main-PCB.

Display Driver: The Main-PCB of the featured LE-81M manufactured in June 1974 makes use of nine Transistors with supporting Resistors as discrete Digit Drivers for the LED display while the TMC0605 chip drives the Segments directly.

Clock: While the nominal clock frequency of the TMS0600 single-chip calculator circuit is specified with 250 kHz, uses the LE-81M a slower pace to reduce overall power consumption of the product slightly. The astable multivibrator using two discrete transistors operates at a frequency between 150 kHz and 250 kHz, we observed with the featured LE-81M manufactured in June 1974 a clock frequency of 180 kHz.

Power Supply: The LE-81M is powered by either a sealed battery pack with four AA-sized rechargeable NiCd cells or a battery holder for four disposable Alkaline batteries resulting in a typical voltage between 4.0 V (completely depleted cells) and 6.0 V (new cells). The Main-PCB hosts a power converter circuit centered around an astable multivibrator, step-up transformer and various diodes and capacitors to generate the supply voltages for the TMC0605 chip and the clock oscillator. We observed in the featured LE-81M manufactured in June 1974 output voltages of V_DD = -7.9 V and V_GG = -15.2 V for the electronics and measured the current consumption with:

The battery packs are compatible with the three first generations of Canon's product portfolio of Portable Handheld Calculators with LED display but don't fit the LE-81:

Keyboard: The keyboard of the LE-81M uses spring-supported plastic keys pushing a small conductive rubber element against two contacts etched on a single-sided phenolic PCB combining both long-travel keys with reasonable manufacturing costs while maintaining longevity of the calculator. The keyboard module is connected with a short 20-pin flat-cable to the Main-PCB.

Canon introduced in 1974 their more cost optimized and last generation of calculators with LED displays:

The next product in Canon's portfolio of Portable Handheld Calculators was the LD-80 sporting a green VFD (Vacuum-Fluorescent-Display) and introduced in 1974, too.

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

© Joerg Woerner, December 5, 2001. No reprints without written permission.