Canon LE-81M

Date of introduction:  1974 Display technology:  LED-stick
New price:  $79.95 Display size:  8
Size:  5.5" x 3.1" x 1.5"
 139 x 78 x 37 mm3
Weight:  6.7 ounces, 190 grams Serial No:  503268
Batteries:  4*NiCd AA or 4*AA Alkaline Date of manufacture:  mth 06 year 1974
AC-Adapter:   Origin of manufacture:  Japan
Precision:  8  Integrated circuits:  TMC0605
Memories:   Displays:  Bowmar Optostic R7H-122-9
Program steps:   Courtesy of:  Peter Muckermann († April 21, 2021)

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 - TMC0605 single-chip calculator circuit
Display Driver - No Segment Drivers and discrete Digit Drivers
Clock signal generation for TMC0605 with discrete components
Power converter with discrete components and transformer
17-pin connector to the Display-PCB
20-pin connector to the Keyboard-PCB

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:

SN75491, SN75492 - Limited to designs with 5 or 6 batteries
TIL360 - Cost driver number one in the calculator design

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:

1972: Two Hermetic Multi-Digit Calculator Numeric Seven-Segment LED Displays soldered onto a PCB
1973: Nine Seven-Segment LED Displays soldered onto a PCB
1974: Nine Seven-Segment LED Display Chips bonded onto a PCB
1975: Nine small Seven-Segment LED Display Chips bonded onto a PCB with additional magnifying lens
1976: LED Displays near extinguished by VFD and LCD technology

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 VDD = -7.9 V and VGG = -15.2 V for the electronics and measured the current consumption with:

Mode Display Current
VBAT = 6.0 V
Clock Frequency
Calculating 0 55 mA 180 kHz
Calculating 88888888 131 mA 180 kHz

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:

1st Generation: LE-10
2nd Generation: LE-80, LE-80M, LE-80R, LE-82
3rd Generation: LE-81M, LE-100, F-5, FC-80

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:

4th Generation: LE-83, LE-84 and LE-85

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.

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