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Keypad Unit The Keypad Unit consists of 4 x 4 matrix keyboard. The Figure 2 shows the keyboard matrix interface to COP888CG. The keyboard is scanned periodically by ad- dressing a column in the keyboard matrix. The program sen- ses the key closure in that column by testing the Port I lines (IO t0 13) which are connected to the rows of the keyboard matrix. Thus, each key is associated with the conjunction of one Port D output line and one Port I input line only. The keypad unit is used to program the real time clock in order to set the time and date. The telephone number to be dialed in case of a security breach can also be programmed through the keypad as well as the terminal keyboard in the Terminal Unit.