Texas Instruments TI-78

Does this rare TI-78 look like an electronic calculator? At first glance you'll notice a numeric keyboard, a multiline display, some promising function-keys but you miss probably all the scientific functions. With a second view you'll probably snatch the alphanumeric overlay of the keyboard and an unusual display with the headline "LOAD MONITOR".

Meditating about the TI-78 type designation places this odd machine somewhere between the TI-74 BASICALC introduced in 1986 and the failed TI-88 scheduled for 1982. If we focus on the style of lettering and the place of the logo we recognize the famous TI-81, introduced in 1990.

And we are right, the TI-78 was introduced in 1990 and continues a tradition started with the TI-59 and carried forward with the TI-88 and TI-74 BASICALC. The keyword in this chain is "customized calculators", the evolution of "programmable calculators".

If you sketch down the block diagram of a calculator you are drawing actually a computer running one fixed program:

Texas Instruments created for the SR-50 a flexible micro-computer architecture for scalable scientific calculators. Just adding one additional Mask-programmed chip to the SR-50 and the outcame was the SR-51 with enhanced statistical functions. Adding one additional RAM as program memory to the design and we got the SR-56. Main disadvantage of this keyboard programmable calculator was the volatile program memory. With the later TI-58 a total of 480 program steps took about 2 hours to be entered with the keyboard. And if your battery was weak the remaining operating time was some 30 minutes. 

The legendary TI-59 - we celebrated May 24, 2007 the 30^th Anniversary - overcame this problem by a card reader for magnetic strips. But the real revolution of the TI-59 was hidden by a small lid on the backside of the calculator, the Solid State Software Modules™ with up to 5000 program steps. Dozens of companies used this module concept for innovative, customized solutions like the Allianz Insurance calculator, the Bossard Screw calculator or the USMC Harrier Flight computer.

The TI-88 even included with the CRAM and CROM Modules two rear slots and allowed with an alphanumerical display and [YES], [NO], and [UNKNOWN]-keys smart dialogs.  

With the TI-74 BASICALC the inconvenient "keyboard code programming" was replaced with the more sophisticated BASIC program language. Once again used Texas Instruments RAM- and ROM-cartridges for this flexible computer system to enhance its capabilities. The almost identical TI-74S was dedicated to OEM applications and found dozens of applications with assurance and insurance companies.

And where is the slot of the TI-78 you may ask? To be honest, it is invisible! The TI-78 went one step ahead and integrated an Infrared Communication port, capable of 38400 bits per second serial communication with a Personal Computer (PC).

Dismantling the TI-78 reveals indeed a well known computer architecture, it is very similiar to the early PC and centered around the INTEL I8088 microprocessor. Other main components of the printed circuit board (PCB) are a ROM manufactured by Mitsubishi with 64k Byte capacity and 8 Sony CXK58257 S-RAM chips with 32k Byte capacity, each. A Toshiba TC17G042 ASIC forms the Glue-logic of the design and connects the peripherals (Keyboard, Display, Communication Ports) to the processing unit. The TC17G ASIC family was manufactured in a 2um C-MOS process and featured complexities between 540 and 10,000 gates. 
The TI-78 was available with 3 different RAM sizes of 256k Byte (shown), 512k Byte and 768k Byte. The upgraded versions made use of 8 resp 12 S-RAM chips with 64k Bytes capacity, each. 
The ROM stores a BIOS (Basic Input-Output System) similiar to an early PC, the application loader interface and the diagnostics and system setup utilities.

The dot matrix display of the TI-78 uses 64 * 120 pixels and shows in the application loader program 8 lines by 20 characters. The visible Toshiba T9842 Display controller drives the rows of the display, the columns are probably connected to the T9841 counter part. We know a similiar display from the first TI-81 introduced in 1990.
The [LIGHT]-key of the TI-78 activates the backlight illumination and enhances the contrast of the LC-Display slightly.

The TI-78 is powered by a slim, rechargable battery pack. We assume an operation time of 10 hours based on the 400 mAh capacity and the internal construction. The S-RAM's maintain the program even if the TI-78 is switched off due to a power saving standby function and an additional CR2025 Lithium battery.

Writing an application program for the TI-78 requires a complete development system based on a DOS 5.0 or DOS 6.0 Personal Computer and a Microsoft C compiler. There are a lot of ideas for such a "Programmable Data Terminal" like inventory control, mail delivery management and more. But even in 1990 there was a lot of competition in this market from Japanese companies.

Texas Instruments sold probably less than 1,000 TI-78 over the short production periode but most of them are still in use!

Peter Reed, Senior Aircraft Structures Engineer of ABX Air, Inc. shared some valuable informations with the Datamath Calculator Museum. 
ABX Air, Inc. an all-cargo airline headquartered in Wilmington, OH with a fleet of over 100 aircraft still uses the TI-78 for calculating airplane weights and balances. ABX Air bought the remaining TI-78 stock when Texas Instruments discontinued it in the 90's. According to Peter are about 300 TI-78 mounted on airplanes and some 200 still in stock. The fleet of ABX Air includes McDonnell Douglas DC-8, DC-9, Boeing 767-200 and 767-200 Special Freighter.



TI-78? Read more about Chinese Lucky Numbers.

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

© Joerg Woerner, May 7, 2007. No reprints without written permission.