Montgomery Ward P200 aka TXI-8662A

Date of introduction:  1974 Display technology:  LED-stick
New price:   Display size:  8
Size:  6.5" x 3.2" x 1.7"
 166 x 81 x 44 mm3
Weight:  7.5 ounces, 214 grams Serial No:  47X-016035
Batteries:  3*AA NiCd or 4*AA Alkaline Date of manufacture:  wk 33 year 1974
AC-Adapter:  AC9130 or AC9120 Origin of manufacture:  USA
Precision:   Integrated circuits:  TMS0601, 2*SN27915, 2*SN27914
Memories:  1 Displays:  DIS134C (9*DISXXX)
Program steps:   Courtesy of:  Joerg Woerner
    Download manual:   (US: 2.8M Bytes)

The Montgomery Ward P200 (TXI-8662A) could be easily identified as a close relative of the TI-2550. Both calculators use the same electronics, the same layout of the keyboard and even an identical mold for the housing bottom. The different look is achieved through a new mold for the upper half of the housing and a different style of the memory keys. The color of the housing is slightly different, too.

Dismantling the featured P200 with Date code 334 manufactured in August 1974 by Texas Instruments in the United States reveals a complex design with three printed circuit boards (PCBs) for main electronics, display, and keyboard powered by either three AA-sized rechargeable NiCd or four disposable Alkaline batteries. 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 - TMS0601 single-chip calculator circuit
• Display Driver - 2*SN27915 Segment Drivers and 2*SN27914 Digit Drivers
• Clock signal generation for TMS0601 with discrete components
• Power converter with discrete components and transformer
• 19-pin connector to the Display-PCB
• 13-pin connector to the Keyboard-PCB

Calculating Unit: The P200 makes use of the TMS0601 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.

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 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 P200 manufactured in August 1974 makes use of a DIS134C Nine-Digit display module with 9 individual DISXXX Seven-Segment displays soldered onto a PCB and magnified with a clear plastic lens. The display module is connected with 19 pins to the Main-PCB.

Display Driver: The Main-PCB of the featured P200 manufactured in August 1974 makes use of a total of four Display Drivers. The two SN27915 (SN75493) Segment Drivers for four segments, each and the two SN27914 (SN75494) 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. We do not recognize the manufacturers of these ICs due to the missing company logos.

Clock: While the nominal clock frequency of the TMS0600 single-chip calculator circuit is specified with 250 kHz, uses the TI-2550 and its sibling P200 the approach of a dynamic switching of the clock frequency for the TMS0601 single-chip calculator circuit to conserve power between calculations. The astable multivibrator idles at a frequency of around 90 kHz but increases with the detection of a depressed keybutton for a short time to about 200 kHz to reduce execution time of the operations. Three diodes are connected between the keymatrix inputs KN (numbers), KO (operations) and KQ (Memory) and the oscillator to catch every entry of a number or function keys for a impressive reduction of power consumption:

Mode Display Current
VBAT = 6.0 V
Clock Frequency
Idle 0 56 mA 90 kHz
Calculating 0 76 mA 200 kHz
Idle E88888888 119 mA 90 kHz
Calculating E88888888 139 mA 200 kHz

A similar circuitry was introduced in August 1973 for the TI-2500 Datamath calculator with the introduction of the TI-2500 Version 3 but using only two diodes for the KN and KO lines. The final TI-2550 Version 2 omitted the dynamic switching of the clock frequency of the TMS0601.

Power Supply: The P200 is powered by three AA-sized rechargeable NiCd or four disposable Alkaline batteries resulting in a typical voltage between 3.0 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 TMS0601 chip and the clock oscillator. We observed in a featured TI-2550 manufactured in March 1974 rather asymmetrical output voltages of VSS = 6.7 V and VGG = -8.0 V for the electronics.

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.

Not only the used LED-modules changed during the life cycle of the TI-2550 calculator, a later cost-reduction redesign introduced a revised Main-PCB and we differentiate between four different TI-2550 Versions manufactured in the United States between February 1974 and October 1975 and their respective Montgomery Ward P200 (TXI-8662A) counterparts:

Version Display
TI-2550 V1D1
P200 V1D1
single modules
 with lens
4 ICs Dynamic
TI-2550 V1D2
P200 V1D2
single chips
without lens
4 ICs Dynamic
TI-2550 V1D3 single chips
with lens
4 ICs Dynamic
TI-2550 V2D3 single chips
with lens
2 ICs Static

Here at the Datamath Calculator Museum we classify the featured Montgomery Ward P200 (TXI-8662A) as PCB Type 1 and Display Type 1, P200 calculators with later manufacturing dates use Display Type 2 instead.

Don't miss a TI-2550 manufactured in October 1975 in Rieti, Italy mixing the latest TI-2550 Main-PCB with a DIS134B Nine-Digit display module preceding the DIS134C located in the very first TI-2550 V1D1 and using the DIS279 Seven-Segment displays from the SR-50 and making it an interesting outlier:

Version Display
TI-2550 V2D0
single modules
with lens
2 ICs Static

Compare it with its scientific sibling P300.

Don't miss the rare TXI-8661A based on the Exactra 23 calculator.

The featured calculator was donated in November 2011 by Scott Brainard. Thank you!

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