Texas Instruments TI-78

Date of introduction:  1990 Display technology:  LCD dot matrix
New price:   Display size:  8 * 20 characters
Size:  5.7" x 3.1" x 0.95"
 145 x 80 x 24 mm3
Weight:  9.8 ounces, 277 grams Serial No:  20000526 B
Batteries:  BP-78 (4.8V NiCd, 480mAh) + CR2025 Date of manufacture:  mth 02 year 1990
AC-Adapter:  none Origin of manufacture:  Japan
Precision:   Integrated circuits:  CPU: OKI M80C88A
 ROM: M5M27C512A
 RAM: 8*CXK58257
 Display: T9842B, T9841B
Memories:  256kB RAM, 64kB ROM    
Program steps:   Courtesy of:  Joerg Woerner

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 or financial 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 release in 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: "Customized calculators", the evolution of "Programmable calculators".

If you sketch the block diagram of a calculator, you are actually drawing a basic computer system running one permanent program:

INPUT: Keyboard
PROCESSING: Mask-programmed processing unit
OUTPUT: Display

Texas Instruments created for the SR-50 a flexible microcomputer architecture for scalable scientific calculators. Just adding one additional mask-programmed ROM (Read-Only Memory) to the SR-50 and the result was the SR-51 with enhanced statistical functions. Adding one additional RAM as program memory to the design and we have the SR-56. Main disadvantage of these keyboard programmable calculators 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 tiny keyboard. And if your battery was weak the remaining operating time was some 30 minutes. 

The legendary TI-59 - we celebrated on May 24, 2007 its 30th Anniversary - overcame this problem with 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 never released TI-88 even included two slots on its rear side to accommodate CRAM and CROM Modules and allowed with an alphanumerical display and its [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 further and integrated an Infrared Communication port, capable of 38,400 bits per second serial communication with a Personal Computer (PC).

Dismantling this TI-78 manufactured in February 1990 by Toshiba in Japan reveals indeed a well known computer architecture, it is very similar to the early PC (Personal Computer) and centered around an Intel 8088 compatible microprocessor manufactured in power-saving CMOS technology by OKI. We identified on the two printed circuit boards (PCBs) five main components plus the battery:

CPU (Central Processing Unit): OKI Semiconductor, now a subsidiary of ROHM, introduced with the M80C88A an officially licensed variant of Intel's 8088 microprocessor using a low-power CMOS process. The -2 marking on its QFP (Quad Flat Package) indicates a maximum speed of 8 MHz.

ROM (Read-Only Memory): ROM is used in computers and other electronic devices to store operating systems, programs and constant data. In mask ROM, the data is physically encoded in the circuit, so it can only be programmed during fabrication. While mask ROM are very economical in large quantities, there are some major disadvantages associated with the technology: The turnaround time between completing the data set for a mask ROM and receiving the finished product is long and bugs lead to a long cycle time.

Subsequent developments addressed these problems and the invention of EPROM, or Erasable Programmable Read-Only Memory, solved the problems, because the memory contend can be reset by exposure the silicon chip through a glass window in the housing to strong UV light. The same EPROM chip packaged into an opaque housing, results in the OTP-ROM, or One-time Programmable Read-Only Memory, a technology usually used for quick production ramp-up at higher costs. The development of the Flash Memory, a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory), allows changes in the programs or data on the fly and replaced mask ROM in most applications.

The featured TI-78 makes use of one M5M25C512AFP chip manufactured by Mitsubishi, Japan, an OPT-ROM with a capacity of 64k Bytes and an access time of 150 ns. The ROM stores a BIOS (Basic Input-Output System) similar to an early PC, the application loader interface and the diagnostics and system setup utilities.

RAM (Random Access Memory): The RAM is used as data memory and is used to store both variables, user programs and intermediate results. The featured TI-78 makes use of eight CXK58257M-10LL S-RAM (Static RAM) chips manufactured by Sony, Japan with 32k Bytes capacity, each. These chips are rated for an access time of 100 ns and selected for low-power standby consumption of only 5 uA. The total capacity with eight memory chips populated amounts to 256k Bytes, but could be expanded to 512k Bytes or 768k Bytes with 16 resp. 24 S-RAMs with 32k Bytes capacity, each.

ASIC: 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.

DISPLAY: The dot matrix display of the TI-78 features 64 * 120 pixels and shows in the application loader program 8 lines of 20 characters, each. The Toshiba T9842B Display controller drives the rows of the display, the columns are connected to its T9841B counter part. We know a similar 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.

POWER: The is powered by a slim, rechargeable battery pack. We assume an operation time of 10 hours based on the 400 mAh capacity and the internal construction. The S-RAMs maintain program and data 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 mainly Japanese companies.

Texas Instruments sold probably less than 1,000 of its TI-78 over the short production period but most of them were as of Summer 2007 still in use!

ABX.jpg (26639 Byte)Peter Reed, Senior Aircraft Structures Engineer of ABX Air, Inc. shared in 2007 some valuable information 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. And don't miss the rare TI-78P Prototype.


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