DATAMATH CALCULATOR MUSEUM |
Radio Shack EC-425
Date of introduction: | June 1974 | Display technology: | LED-stick |
New price: | $79.95 | Display size: | 8 + 2 |
Size: | 6.1" x 3.1" x 1.5" 156 x 78 x 38 mm3 |
||
Weight: | 9.2 ounces, 262 grams | Serial No: | 010737 |
Batteries: | 3*AA NiCd | Date of manufacture: | wk 31 year 1974 |
AC-Adapter: | AC9130 | Origin of manufacture: | USA |
Precision: | 8 | Integrated circuits: | TMS0120 (TMS0720), 2*SN75493, 2*SN27423 |
Memories: | Displays: | DIS175 | |
Program steps: | Courtesy of: | Joerg Woerner |
This
EC-425 distributed by Radio Shack (A Tandy corporation company) could be easily
identified as a restyled SR-10. The mold of the housing looks very similar to other restyled
Wedge area calculators like the Montgomery Ward P200.
Dismantling
this Radio Shack EC-425 with Date code 314 manufactured in
August 1974 by Texas Instruments in the United States reveals indeed a printed
circuit board (PCB) with a prominent SR-10 marking. The complex design of the
EC-425 with three
printed circuit boards (PCBs) for main electronics, display, and keyboard
powered by three AA-sized rechargeable NiCd batteries resembles Version 2 of the
SR-10 but makes use of a different
LED display assembly mounted at a steeper angle for better readability. The Main-PCB sports not
only five familiar looking Integrated Circuits (ICs) but a myriad of discrete
components in a from the Datamath well known arrangement:
Calculating Unit - TMS0120 single-chip calculator circuit Display Driver - 2*SN75493 Segment Drivers and 2*SN75423 Digit Drivers Clock signal generation for TMS0120 with discrete components Power converter with discrete components and transformer 21-pin connector to the Display-PCB 15-pin connector to the Keyboard-PCB |
Calculating Unit: The
Radio Shack EC-425 makes like its sibling SR-10 use of the TMS0120 single-chip calculator circuit derived from the TMS1802,
better known as first "calculator-on-a-chip". Around July 1973 the first TMS0100 designs were ported to an 8-micron process and internally renamed to
TMS0700 but still marked on the outside of the package with TMS01XX. The
featured Radio Shack EC-425 manufactured in August 1974 uses according to its
marking on the bottom of the package a TMS0720.
Display:
The featured EC-425 manufactured in August 1974 uses a
DIS175 Twelve-Digit display module based
on Bowmar's Optostic technology opposed to
the
DIS115F Twelve-Digit display module with 12 individual DISXXX Seven-Segment displays and integrated magnifying lens
used in the SR-10 Version 2. Please notice that the DIS175 display module has its left
most character only partly populated for Overflow indicator/negative sign
Mantissa and the character used for negative sign Exponent sports just the
corresponding segment.
Display Driver: The Main-PCB of the featured
EC-425 manufactured in August 1974 makes use of a total of four Display Drivers. The two
SN75493 Segment Drivers for four segments, each and the two
SN75423 Digit Drivers for six digits, each are improvements of the original
SN75491/SN75492
chips introduced with the TMS1802 but allow for operation at lower voltages.
Clock: While the nominal clock frequency of the TMS0100 single-chip calculator circuit is specified with 250 kHz, uses the SR-10 a slightly reduced 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 200 kHz, we observed with the featured EC-425 manufactured in August 1974 a clock frequency of 166 kHz.
Power Supply: The EC-425 is powered by three AA-sized rechargeable NiCd batteries resulting in a typical voltage between 3.0 V (completely depleted cells) and 4.5 V (while charging full 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 TMS0120 chip and the clock oscillator. We observed in the featured EC-425 manufactured in August 1974 rather asymmetrical output voltages of VSS = 7.3 V and VGG = -8.2 V for the electronics.
Battery Saver Circuit: To save battery power the LED display turns off automatically between 15 and 60 seconds after the last keyboard entry, except for the first digit (Digit D3 of TMS0120, LSD of Mantissa). If the display turns off while entering a problem, the display turns on automatically with the first keyboard entry. Depressing the [=] key brings back the last calculated display. Three diodes are connected between the keymatrix inputs KN (numbers), KO (operations) and KP ([1/x], [x2] and [sqr X]) and a simple monoflop to catch every entry of a number or function keys to keep the Digit Drivers enabled. If the monoflop time expires, the Digit Drivers sans Digit 3 are disabled for an impressive reduction of power consumption:
Mode | Display | Current VBAT = 4.5 V |
Clock Frequency |
Calculating | 0 | 52 mA | 166 kHz |
Power Save | 0 | 52 mA | 166 kHz |
Calculating | E88888888-88 | 127 mA | 166 kHz |
Power Save | 8 | 72 mA | 166 kHz |
Keyboard: The Klixon type keyboard looks very similar to the Datamath calculator with some additional keys placed in the upper line. Later calculators like the SR-11 changed the style of the keys but kept the extreme wedge-style of the housing that was adopted for the scientific desktop calculators SR-20 and SR-22, too. The last portable scientific calculator with this wedge-design was introduced in October 1974 with the SR-16.
And how does the Radio Shack EC-425 fit with its Bowmar Optostick 175 LED display into the known four different SR-10 Versions manufactured in the United States between November 1972 and June 1975? Next to the Montgomery Ward P300 Version 2:
Calculator | Position of logo |
Display Type |
Display Driver |
SR-10 USA V1 | Display frame | 6-digit modules without lens |
4 ICs |
SR-10 USA V1D2 | Display frame | single modules with lens |
4 ICs |
SR-10 USA V2 | Keyboard | single modules with lens |
4 ICs |
Montgomery Ward P300 V1 |
None | single modules with lens |
4 ICs |
Montgomery Ward P300 V2 |
None | LED-stick w/o lens |
4 ICs |
Radio Shack EC-425 |
None | LED-stick w/o lens |
4 ICs |
SR-10 USA V3 | Keyboard | single modules with lens |
2 ICs |
Here at the Datamath Calculator Museum we classify the featured Radio Shack EC-425 as Display Frame OEM, PCB Type 2 and Display Type 3.
If you have additions to the above article please email: joerg@datamath.org.
© Joerg Woerner, December 5, 2001. No reprints without written permission.