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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.