Texas Instruments TI-85 ViewScreen™

Texas Instruments Inc. filed already September 27, 1991 the US Patent Application U.S. Pat. No. 5,168,294 with the title “Display demonstration system for providing enlarged projected image of calculator display”. It addresses three different problems found with the previous approach invented by William T. Stokes and published on July 30, 1991 as US Patent Application U.S. Pat. No. 5,035,502 with the title "Transparent calculator for overhead projection". 

In principle Texas Instruments connected with a long cable a second, transparent remote LCD display to the existing TI-81 graphing calculator and used a raised platform with an additional lens to enlarge the image while shielding the remote display from excessive heat. The used platform was in the 90s commercially available from the Polaroid Corporation to project Polaroid color-graph type 691 overhead transparencies.

The TI-85 ViewScreen is the teacher version of the standard TI-85. It connects with a special port permanently to a ViewScreen panel via a flat-ribbon cable. Placing the panel on the overhead projector enlarges the image of the handheld screen so that each student can follow along. From a technical point of view the TI-85 VSC is almost identical with the students TI-85. The main difference you notice is a two piece bottom shell of the calculator housing to accommodate the ViewScreen electronics.

Dismantling the TI-85 VSC reveals a small printed circuit board (PCB) with one driver chip connected in a piggy back manner to the display board of the calculator. 

The main electronics of the TI-85 VSC is identical with the students model TI-85 and centered around the well-known Toshiba T6A43 Application Specific CPU, the 8k Bytes Static RAM chip used in the TI-81 was replaced with a 32k Bytes chip and the capacity of the ROM (Read-Only Memory) was doubled from 64k Bytes to 128k Bytes.

It makes use of just six main components on the printed circuit boards (PCBs):

CPU (Central processing Unit): The Toshiba T6A43 is a so-called Application Specific CPU and combines a Z80 core with an ASIC (Application Specific Integrated Circuit). You’ll find different approaches within the graphing calculator line of Texas Instruments, sometimes the use of an individual CPU (e.g. Z80 and M68000) with a supporting ASIC or – like with this TI-85 VSC - the integration of the CPU into the ASIC. Learn more about the Hardware Architecture of TI’s Graphing Calculators.

ROM (Read Only Memory): The ROM contains the operating system of the calculator. The first products on the market used mask-programmable ROM’s, the program was stored already during the production of the Integrated Circuit. Later calculators changed to Flash-ROMs, a technology allowing the programming of the software during the final production stage of the calculator. With the TI-83 Plus and all later graphing calculators from Texas Instruments even the user was able to reprogram the operating system. 

This TI-85 VSC manufactured already in June 1992 makes use of a OTP-ROM (One-time Programmable Read-Only Memory) Toshiba TC541001AF. Later TI-85 calculators host a Mask-ROM LH531 manufactured by Sharp,  Japan. It is common practice in software engineering to avoid the high one-time costs and long lead time of a Mask-ROM during the ramp up of a new product. One-time programmable ROM like the TC541001 are on the other hand more expensive.

RAM (Random Access Memory): The RAM is used as data memory and is used to store both variables, user programs and intermediate results. This TI-85 VSC makes use of SRM20256 manufactured by Suwa Seikosha, Japan. The capacity of the memory is 32k Bytes.

DISPLAY: The two Toshiba T6A39 are column drivers for small-to-medium-sized dot matrix graphic LCD’s, while the T6A40 is a row driver. They are compatible with Z80 based CPU’s and drives displays with up to 80 columns, resp. 68 rows. The display size of the TI-85 is 128 * 64 pixel, therefore a total of three drivers are necessary.

Texas Instruments revised the concept of the ViewScreen in 1994 and even 15 years later were four ViewScreen panels available:

As an alternative the TI-Presenter video adapter connects to a TV or other projection device with a video input port. 

ROM-Versions:

3.0A

You can check the ROM version of your TI-85 VSC using the following key sequence and reading the number on your screen:

[2nd] [MODE] [ALPHA] [S]

Information provided by ticalc.org and Xavier Andréani.

Exam acceptance:

The TI-85 VSC is permitted (as of September 27, 2007) for use on SAT, ACT, PSAT and AP exams.

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

© Joerg Woerner, February 23, 2009. No reprints without written permission.