TI-55

The TI-55 is a programmable calculator first manufactured by Texas Instruments in 1977. It has an LED display, and weighs 6.4 ounces (180 grams). It is programmable to hold up to 32 key-codes that allow the user to repeat simple calculations with different values.
Variants.
TI-55 II.
The TI-55 II with LCD was introduced in 1981, but, like many other Texas Instruments calculators of this time, suffered from serious keyboard reliability problems. Several variants of the TI-55 II exist.
TI-55 III.
The TI-55 III replaced the TI-55 II in the USA in 1986. It features redesigned keyboard mechanics, thereby eliminating the common "bouncing keys" fault of prior models. Several variants of the TI-55 III exist.
TI-56.
The TI-56 is a European variant of the TI-55 III manufactured since 1986. It was also offered free of charge in exchange of faulty Texas Instruments calculators of a similar class like the TI-54 in Europe in 1986.