Texas Instruments TI-88 (PVT 4)

We wrote May 1982 when Texas Instruments printed the sales brochures of the TI-88 on glossy paper. The brochures promised the introduction of the TI-58C/TI-59 successor for Forth Quarter 1982 in a package matching the TI-55-II line - before we got word of the TI-66. Today we know more: The development of the TI-88 was finalized, the manuals printed, the first pre-production calculators worked perfectly and Texas Instruments canceled the whole project! Maybe one of the reasons was the introduction of the powerful Hewlett-Packard HP-41C calculator or the innovative Sharp PC-1211 Pocket Computer. Or the underestimated complexity of the TI-88 design. Or electrostatic discharge (ESD) issues of the Module contacts. Or the internal competition with the CC 40 Compact Computer. Or the keyboard problems encountered with the TI-55-II family of calculators. Or the decision of a Manager of TI's Consumer Electronic group to quit the calculator business...

Learn more about the demanding "Project X" initiated already in Summer 1977 and leading not only to the (failed) TI Programmable 88, but the TI-66 PROGRAMMABLE, failed TI-76 PROGRAMMABLE, Compact Computer CC 40, (failed Compact Computer CC 70) and last but not least the successful TI-74 BASICALC and TI-95 PROCALC.

As of today we researched almost twenty different TI-88 calculators with twelve different Builds or modifications (learn more about the Five Engineering Stages):

Just looking at the keyboard of the TI-88, you can easily differentiate between the three Engineering Stages identified so far:

Please be aware that the faceplates of most of the surviving TI-88s were updated to the Production Validation Test design and the most reliable indicator of the Three-and a-Half Main Engineering Stages can be found with the Revision Number located on the printed circuit board (PCB):

If you are interested in the subtle differences between the ten different builds discovered as of today, please follow this link.

Compared with the TI-58C you'll notice some differences and similarities:

It is a pity that the TI-88 with the perfect prompting system never made it to the market. The next calculators in the "Programmable/Scientific line" are the TI-95 PROCALC and the first Graphing calculator TI-81.

A Texas Instruments price list printed in July 1982 suggested these MSRPs:

Inside the TI-88

In 2002 the Patent application US4,447,881 filed by Texas Instruments in 1980 was discovered by Juergen Dobrinski and in 2020 a binder labeled "Project X" including schematics of a "Product 225 TI-85" from the estate of CB Wilson was located by Jon Guidry and made available on his website www.hexbus.com. Together with hours and hours of reverse engineering based on the inside views of the five different TI-88 builds discovered so far and looking into various CRAM and CROM Modules, we understand the TI-88 calculator architecture pretty well. The design is centered around a 4-bit microcontroller of the TP0485 family called Master Controller with an associated TP0485 4-bit controller responsible for Timekeeping, Key Scan and I/O functions supported by both ROM (Read Only Memory), RAM (Read/Write Memory) and Display Drivers. We know a similar approach with two controllers already with the TI-55 II.

Dismantling this TI-88 from the final PVT 4 (Production Validation Test 4) series manufactured in August 1982 by Texas Instruments in their Abilene, TX facility, reveals a design using the following main components:

The Schematics Diagram from the Patent application of "Product X", leading over "Product 225 TI-85" to the final TI-88 design, omits unfortunately the chip numbers but we could reverse engineer them accordingly.

MCU (Microcomputer Unit): The TI-88 is based on two TP0485 chips, a complete reversal of the legendary TMC0500 chip set for scientific calculators introduced in 1975 with the SR-50 and used for TI’s flagship calculators TI-59 and SR-60A till the mid of the Eighties and its spiritual successor TMC1500 used with the TI-57. While the TMC0501 Arithmetic Chip needed in a minimum configuration one extra TMC0520/530 SCOM (Scanning and Read Only Memory) or TMC0580 DSCOM (Scanning and Read Only Memory with double capacity) chip, integrated the LCD III microcontroller family both RAM and ROM allowing a single-chip configuration. The “register processor” architecture of both the TMC0500 and TMC1500 was highly optimized for arithmetic calculations used with scientific calculators and lacked flexibility for performing other tasks. The “digit processor” architecture of the LCD III on the other hand is based on a more flexible 4-bit architecture derived from the TMS1000 but optimized for fast processing speed and high modularity.

The TP0485 is a member of the LCD III family and the final evolution before the design was cancelled together with the TI-88:

These chips can be bonded either in a 0.4” wide 28-pin SPDIP (Shrink Plastic Dual In-line Package with a 0.07” / 1.778 mm lead pitch) or a 0.6” wide 40-pin SPDIP encapsulation.

The planned product portfolio of the LCD III family included devices with smaller ROM capacity (1k*13 Bits or 2k*13 Bits) and RAM capacity intended for use in different products like the TI-55 II or even simple 4-banger calculators like the TI-1030, but these chips never materialized.

The TI-88 divides the various software and hardware tasks into the TP0485/CD2902 Master Controller (sometimes called Memory Controller) with a 28-pin package and a TP0485/CD2901 Timekeeping, Key Scan and I/O Controller with a 40-pin package.

• TP0485/CD2901

• TP0485/CD2902

Communication between the two TP0485 microcomputers and additional RAM and ROM devices is realized with a bidirectional 4-bit bus with up to 266k Bit/s while communication to the dot matrix LC-Display is using a serial approach. The peripheral I/O connects either to external devices like printer and cassette interface or another TI-88 calculator with a serial, bidirectional bus allowing between 1.2k Bit/s and about 2.0k Bit/s.

CROM (Customer Read Only Memory): The TI-58 and TI-59 programmable calculators introduced a novelty, the Solid State Software Modules with up to 5,000 program steps that could be inserted by the user through an opening called “port” on the backside of the calculators and covered by a small lid. The Master Library module was included with the calculators, 12 additional SSSM covering various applications were available from Texas Instruments and probably 50 to 100 modules from third-party companies.

The TI-88 continues this approach but expanded it in three dimensions:.

This TI-88 manufactured in August 1982 makes use of one TP0532/CD5402 chip with a capacity of 120,000 Bits organized as 15,000 Bytes and using a bidirectional 4-bit bus to communicate with the TP0485/CD2902 Master Controller. While only 10 pins are connected to the outside world is the TP0532 CROM packaged in a 0.4” wide 28-pin SPDIP housing.

The TI-88 architecture uses a very clever approach of distributing program code in the different ROMs:

• TP0532/CD5402

CRAM (Custom Random Access Memory): This TI-88 manufactured in August 1982 makes use of two TP0531 chips with a capacity of 4,800 Bits or 600 Bytes, each – more than twice the capacity of the TMC0598 PRAMs used with the TI-58/59 family. The TP0531 is using a bidirectional 4-bit bus to communicate with the TP0485/CD2902 Master Controller. While only 10 pins are connected to the outside world is the TP0531 CRAM – like the TP0532 CROM – packaged in a 0.4” wide 28-pin SPDIP housing. Both chips have an almost identical pin-out differing only in the position of the respective Chip-Select pins, this allows to solder one CROM and one CRAM in a piggy-back way on top of each other.

This TI-88 is hiding one of the two TP0531 chips under the visible TP0532/CD5402 chip with the other TP0531 next to it.

DISPLAY: The TI-88 makes use of a dot matrix LC-Display with 16 characters supporting a 7x5 pixel font, each and a some extra status symbols. The LC-Display itself with its 1034576-4 marking was obviously custom manufactured in Japan for Texas Instruments. The Date code "1D4" of the LC-Display Module from a TI-88 "PVT 3 Build" with Series Number 1073 translates to 4th week in April 1981.

Texas Instruments developed with the TP0530 a cascadable display driver with 40 + 1 column drivers and 7 row drivers. Two of these drivers are used in the TI-88, each driving one half of the LC-Display. The larger of the two PCBs with the "M" marking generates the row-scanning of the LC-Display and connects with 48 pads to it while the smaller PCB with the "S" marking sports just 42 pads.

The Integrated Circuits with a total of 63 connections are bonded directly in Chip-on-Board (COB) technology on the printed circuit board and covered with a black epoxy blob.

The block diagram of the TP0530 provided by Jon Guidry reveals an interesting structure with a pure serial approach, the CPU needs to clock-in 8 characters of 8 bits, each and 3 bits for the status information for each half of the display. The characters are converted with an internal 7*5 font generator ROM into the 40 column signals while a free running counter address the 7 rows of the display each. The 3-bit status information is simply reflected on one additional column driver for a maximum of 7 different status signals for the user. The Master chip is responsible for the scanning of the display and generates the timing for the cascaded TP0530 chips.

POWER SUPPLY: We assume that all Integrated Circuits were manufactured in a low-power CMOS process, nevertheless used Texas Instruments once again a rechargeable battery pack labeled accordingly BP88.

Instead of the 3 AA-sized NiCd cells of the TI-59 introduced in 1977, used this design of 1981 just one AA-sized cell and a step-up converter SN77203 to generate both the supply voltage of the Integrated Circuits and LC-Display. The battery is recharged with an AC9133 AC-Adapter, technically very similar to the AC9132 used with the TI-30 and other calculators but using a different connector.

ALGORITHM: Running Mike Sebastian's "Calculator forensics"  gives a result of 9.000000955917. It took 7 years before with the TI-68 another Texas Instruments calculator achieved a similar precision.

CRAM and CROM Modules

Two different type of modules were designed:

• CRAM-Modules as user programmable memory with either 1184 program steps or 148 user memories.

Dismantling a CRAM-Module from the final TI-88 PVT 4 Build reveals two TP0531 Read/Write Memories with 4,800 Bits or 600 Bytes capacity, each and a Sanyo 3 Volts lithium battery with 24.5 mm diameter and a thickness of 3.5 mm - matching today's CR2430 cells.

  • CROM-Modules as pre-programmed software like the earlier TI-58C/TI-59 Solid State Software™ modules with up to 15,000 program steps.
   Eight modules were already defined in the sales brochures:

Dismantling a Master Library CROM-Module from the final TI-88 PVT 4 Build reveals one TP0532 Read Only Memory with Custom Software CD5403 with 120,000 Bits capacity organized as 15,000 Bytes.

Lucky Numbers?

People in China traditionally associate luck with numbers. As a rule in day-to-day life in China, it is customary to regard even numbers as being more auspicious than odd numbers. Eight, 'Ba' in Chinese has a similar sound to 'Fa', which means to make a fortune. All business men favor this number very much. However, for Texas Instruments the "8" in the type designation was not always a fortune.

We remember some very unlucky calculators:

History repeats - don't miss the story about the PET Project!

The TI-88 is featured in the Texas Instruments Announcement dated May 1982.

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

© Joerg Woerner, May 11, 2007. No reprints without written permission.