Texas Instruments TI-2550 (1^st design, Display 2)

Texas Instruments Introduced on January 10^th, 1974 with the TI-2550 their first basic four-function calculator with independent memory and despite its short lifecycle of only 18 months went the calculator through various redesigns to lower its manufacturing costs.

Dismantling the featured TI-2550 manufactured in December 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: The TI-2550 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.

Comparing the feature sets of the TMS0119 (TI-2500), TMS0601 (TI-2550), TMS0120 (SR-10) and TMS0602 (SR-11) shows the limitations of the TMS0100/TMS0600 and explains the move from Texas Instruments towards architectures with scalable ROM configurations like the TMS0200 Building Blocks for Desktop Calculators introduced in 1973 but most important to the TMC0500 Building Blocks for Scientific and Programmable Calculators introduced with the "Slide Rule" calculator SR-50 in January 1974 and leading all the way to the legendary TI Programmable 59 and the amazing SR-60A Prompting Desktop calculator:

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:

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:

The featured TI-2550 manufactured in December 1974 makes use of a DIS206A Nine-Digit display module based on Bowmar's Optostic technology with 9 Seven-Segment displays chips bonded onto a PCB and protected with a red plastic lens. The display module is connected with 19 pins to the Main-PCB.

Display Driver: The Main-PCB of the featured TI-2550 manufactured in December 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 but learned that Bowmar had for a brief period their own display drivers marked as 502-5 (27914) and 503-5 (27915).

Clock: While the nominal clock frequency of the TMS0600 single-chip calculator circuit is specified with 250 kHz, uses the TI-2550 Version 1 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:

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 TI-2550 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 V_SS = 6.7 V and V_GG = -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:

Here at the Datamath Calculator Museum we classify the featured TI-2550 as PCB Type 1 and Display Type 2.

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:

A close relative of the TI-2550 was sold through the American department store Montgomery Ward. Compare the TI-2550 with the P200.

The same housing and a very similar keyboard was used by the TI-150.

Datamath™ is a trademark of Texas Instruments.

The TI-2550 is featured in the Texas Instruments Deutschland GmbH leaflet ER-1975 dated 1975.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, October 23, 2003. No reprints without written permission.