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HP-28C and HP-28S Calculators

The HP-28C and HP-28S have built in internal, display, and keyboard test functions.A continuous internal/display test is performed by simultaneously pressing the ON key and the left-arrow key (labeled "E" in the illustration at the right). Various internal and display tests will be performed until interrupted by simultaneously pressing the up-arrow key (labeled "C" in the illustration at the right) and the ON key.The more thorough internal/display/keyboard test is performed by pressing the ON key and the down-arrow key (labeled "D" in the illustration at the right). Press any key to step through the various display and internal tests until "KEYBOARD TEST" is displayed. Starting with the left keyboard, in order press each key from left to right on each row, from the top row to the bottom row. Then proceed to press each key on the right keyboard in order from left to right and top to bottom. If the test is successful, "OK" followed by the model number will be displayed. If a key is pressed out of order or there is a keyboard problem, and error message will be displayed. This test can be interupted at any time as described above.
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