Montgomery Ward P300 aka TXI-8663A (Version 2) 

This Montgomery Ward P300 aka TXI-8663A could easily identified as larger brother of the P200 aka TXI-8662A and a close relative of the SR-10.

Dismantling this Montgomery Ward P300 with Date code 454 manufactured in November 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: The Montgomery Ward P300 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 Montgomery Ward P300 manufactured in November 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 P300 manufactured in November 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 P300 manufactured in November 1974 a clock frequency of 172 kHz.

Power Supply: The P-300 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 P300 manufactured in November 1974 rather asymmetrical output voltages of V_SS = 7.0 V and V_GG = -8.0 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:

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 Montgomery Ward P300 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 Radio Shack EC-425:

Here at the Datamath Calculator Museum we classify the featured Montgomery Ward P300 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, November 20, 2021. No reprints without written permission.