Texas Instruments TI-1250 (Version 1)

Date of introduction:  June 1, 1975 Display technology:  LED-stick
New price:  $24.95 Display size:  9
Size:  5.5" x 2.8" x 1.4"
 138 x 70 x 35 mm3
Weight:  3.9 ounces, 110 grams Serial No:  0536382
Batteries:  9V  Date of manufacture:  wk 34 year 1975
AC-Adapter:  AC9180 Origin of manufacture:  USA (LTA)
Precision:  8 Integrated circuits:  TMS0952
Memories:  1    
Program steps:   Courtesy of:  Joerg Woerner
    Download manual:   (US: 3.1M Bytes)
  (EU: 3.5M Bytes)

Introduced about three month after the TI-1200, the TI-1250 was the "fully loaded" design adding both a 4-key Memory and a [CS] or [+/-] key to the feature set. Basically both calculators were identical, most important was the higher sales margin of the TI-1250 during the Calculator War due to the additional features.

The later TI-1450 was nothing else than a cosmetically enhanced version of the TI-1250.

The only known models in the TI-1200 family with rechargeable batteries instead the 9V battery are the rare TI-1205, TI-1255 and the limited function calculator ABLE developed for educational purposes.

Read more about power consumption of electronic calculators here.

Unfortunately this cheap calculator killed the TI-150.

Dismantling the featured TI-1250 with Date code 3475 LTA and manufactured in August 1975 in Lubbock, Texas reveals a very efficient and cost-optimized design with a single-sided printed circuit board (PCB) centered around a TMS0952 single-chip calculator circuit based on the TMS1000, the World's first Microcomputer. Compared to earlier designs like the TMS0100 or TMS0800, the TMS0952 integrated both the segment and digit drivers for a 9-digit LED display and clock circuitry, resulting in just 6 discrete components (4 resistors, 1 capacitor and [ON-OFF] switch) on the PCB.

This original design of the TI-1200/TI-1250 was only manufactured for a few months, Texas Instruments changed the PCB slightly to accommodate four additional 330Ω resistors to pull down the four keymatrix input lines of the TMS0952 Chip to VDD. The appearance and functionality of the revised TI-1250 did not change, we observed some TI-1250 manufactured around February 1976 without these resistors, though.

The TMS0952 was soon replaced with the TMS0972, a pin-compatible design dropping the additional resistors and capacitor and further reducing the manufacturing costs of the TI-1250 series. While dropping a few discrete components from the PCB doesn't sound too impressive, did the little improvements here and there add up quickly. With the Calculator War starting in 1975, manufacturing costs of basic 4-function electronic calculators were under extreme pressure and the calculator chip was one of the main cost drivers. The manufacturing costs of an Integrated Circuit (IC) are calculated with:

IC cost = (Die cost + Testing cost + Packaging cost) / Final test yield

With the die cost roughly proportional to the die area, testing and packaging costs roughly proportional to the pin count, and the final test yield mostly inverse proportional to the die area, goals are well defined: Keep the die size as small as possible for a set of requirements agreed on. While the original design of the TMS0952 resulted in a die size of approximately 200 mils * 210 mils / 5.1 mm * 5.3 mm, was Texas Instrument's engineering team able to shrink the die of the TMS0972 - while keeping its functionality - to 190 mils * 160 mils / 4.8 mm * 4.0 mm or a reduction of almost 30%.

Another contribution of cost cutting had a side effect for the customer, Texas Instruments decided to use with the TMS0972 an 8-digit LED display instead of the previous 9-digit display. While the 9th (leftmost) digit was originally used only for the negative sign and didn't impact most calculations, should you try this example: 

11111111 [-] 23456789 [=] - TMS0952: -12345678
11111111 [-] 23456789 [=] - TMS0972: -.1234567 (flashing)

Please notice that the TI-1265 wasn't effected by these changes, this calculator is based on the TMS1043 single-chip calculator circuit shared with the TI-1650.

The final step of cost reduction of the TI-1200/TI-1250 family was introduced just a few months before its discontinuation and included a smaller encapsulation of the TMS0972 Chip. While the original design was using a 0.6 wide 28-pin DIP (Plastic Dual In-line Package with a 0.1 / 2.54 mm lead pitch) encapsulation, switched the final design to a 0.4 wide 28-pin SPDIP (Shrink Plastic Dual In-line Package with a 0.07 / 1.778 mm lead pitch) encapsulation.

Summarizing the internal design of the TI-1200/TI-1250 calculator family, we differentiate between four different printed circuit boards and five different versions:

Version PCB
Date codes
Date codes
Display Discrete
TI-1250 V1 Type 1 2275 LTA 3375 LTA TMS0952NL
DIP28 9-digits 4 Res, 1 Cap
TI-1250 V2 Type 2 3475 LTA 0876 LTA TMS0952NL
DIP28 9-digits 8 Res, 1 Cap
TI-1250 V2 CR Type 2 0876 LTA 0876 LTA TMS0952NL
DIP28 9-digits 4 Res, 1 Cap
TI-1250 V3 Type 2 1276 LTA 1277 LTA TMS0972NL
DIP28 8-digits 1 Jumper Wire
TI-1250 V4 Type 3 1577 LTA 1877 LTA TMS0972NL
SPDIP28 8-digits  
Type 4 1676 LTA 2778 MTA TMS0972NL
DIP28 8-digits Step-up Converter

Please notice that the above table lists only TI-1200/TI-1250 calculators manufactured in the Lubbock, Texas facility, both calculators were actually manufactured in Brazil, Hong Kong, Italy and Spain, too but design changes were typically introduced in the US facilities first.

Texas Instruments did not only change the internal pieces of the TI-1200/TI-1250 over time, even the Product Labels on the rear of the calculators evolved over the years and we differentiate for the US version between three different styles and number ranges (Examples for TI-1250) plus a special label used for repairs:

Style 1 (3775 LTA - 0376 LTA): Texas Instruments TI-1250, Serial Number: 1250 nnnnnn printed black on gold
Style 2 (0376 LTA - 0876 LTA): Texas Instruments TI-1250, Serial Number: 1250 nnnnnnn printed gray on tan or black on gold
Style 3 (3475/0976 LTA - 3976 LTA): Texas Instruments TI-1250, Serial Number: nnnnnnn printed gold on black
Style 4 (4276 LTA - 1877 LTA): Texas Instruments, Serial Number: A nnnnnnn printed gray on sand
Style 4 (4576 LTA - tbd LTA): Texas Instruments, Serial Number: B nnnnnnn printed gray on sand
Style 5: Texas Instruments, Repair Label, no Serial Number

Some members of the TI-1200/TI-1250 family - like the colorful TI-1255 - sport different labels, we identified so far 3½ additional designs:

Style 3W: Texas Instruments Spirit of '76, Serial Number: S76-nnnnnn printed blue on white
Style 6: Texas Instruments TI-1255, Unique style, printed gray on beige
Style 7: Texas Instruments TI-1270, Unique style, printed gray on beige
Style 8: Texas Instruments ABLE, Unique style, printed gray on white

Here at the Datamath Calculator Museum we classify the featured TI-1250 as Hardware Version 1, PCB Type 1 and Product Label Style 3.

Please find the label of a TI-1250 manufactured in Hong Kong here

The Dutch department store HEMA sold an identical model as reactor 1250. A similar model was sold by Western Auto as the electronic Wizard M4987 and True Value Hardware with the model T-1225.

Texas Instruments sold the wildly successful TMS0972 single-chip calculator circuit to other calculator manufacturers, too and with National Semiconductor even a competing calculator chips company used it for one of their own products. Explore the NS835A, Bohsei 3000, Conic EL-601 and Privileg 842M.

Don't miss the Sharp EL-204 calculator, changing the 6x4 layout of the keyboard into a 5x5 matrix with an additional sqr(X) function.

Press the X-RAY button and view the internals of a TI-1250.
 (Pictures provided by Edward Soudentas)

The TI-1250 is featured in the Texas Instruments Deutschland GmbH leaflets ER-1975 and ER-1976 dated 1975 resp. 1976.

horizontal rule

If you have additions to the above article please email:

Joerg Woerner, January 4, 2002. No reprints without written permission.