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Texas Instruments TI-3500

Updated 4/22/2001

The TI-3500 is a slightly more capable version of a stablemate machine, the TI-3000. The 3500 offers two more digits of capacity than the 3000, selectable fixed (2 or 4 digits behind the decimal point) or floating decimal point, as well as providing a constant mode for multiplication and division.

Circuit Board Detail (showing main IC, power supply and display driver circuitry)

The 3500 utilizes a TI-made TMS0106 calculator chip rather than the TMS0109 used in the TI-3000. The TMS0100-series IC's all shared the same basic logic, with mask-programmable ROM and logic arrays that would allow the chip to be tailored to different applications. The rest of the circuitry in the 3500 is very similar to that in the 3000, with the major differences being the calculator chip itself, and a few extra transistors for driving the two extra digits of display that the 3500 provides. This particular 3500 was made in the late part of 1973, based on date codes on numerous components in the machine. The display is a Panaplex-style display with eleven digit positions, with the left-most digit position used for error and sign indication. The display is driven by discrete transistor (TI 2N5400) circuitry. The display module has comma capability in the left-most 8 digits, with decimal point only in the right-most 3 digits. In operation, the commas light at every third digit in front of the decimal point to ease in reading larger numbers.

The Panaplex-style Display Module

The 3500 indicates an overflow condition slightly differently than the 3000. The 3000 gives an "E" indication, which while mnemonic for "Error", precludes indicating the sign of the overflow. The 3500 opts to display a 'u' when the machine overflows, which shows up as an 'o' if the overflow is negative. Overflow, or divide by zero situations lock the keyboard until the "C" button is pressed to clear the machine.

Like the TI-3000, the 3500 uses a sealed keyboard assembly, utilizing a conductive pad on the key stalk that contacts traces on a circuit board under the keypad when the key is depressed. This type of keyboard is very difficult to service, and was likely intended to be the point of failure on these machines, prompting the machine to be thrown out and a new calculator purchased when the keyboard wore out.

The 3500 calculates just a little slower than the 3000, likely because it is working with two extra digits. The ubiquitous 'all nines' divided by one taking just about 1/3 of a second to complete. The display is not blanked during calculation, and the digits jump about a bit when longer calculations are performed.

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