Display and keypad, diskey board. At the center of the diagram, and the heart of the circuit is the P I C 16C57C integrated circuit. This is a 28 pin DIP (dual inline package). Pins 1, 2, and 28, are tied together and go to plus 5 volts. Pin 3 is not connected. Pin 4 goes to ground. Pin 5 is not connected. Pin 6 goes to pin 5 of the 24LC018, or maybe it's 01B. Pin 7 goes to pin 4 of the LCD board. Pin 7 also goes to pin 6 of the 24LC018. Now the P I C 16C57C pins connect to the LCD pins as follows. 8 to 5, 9 to 6, 10 to 7, 11 to 8, 12 to 9, 13 to 10, 14 to 11, 15 to 14, 16 to 13, and 17 to 12. The keypad switches are in an array 3 wide and 4 high. There are 4 horizontal wires running across the array and one end of each switch is connected to the wire closest to that switch. There are three vertical wires and the other end of each switch is connected to the nearest wire. For example the switch for the number one is connected between row 1 and column 1. The switch for the number 2 is connected between row 2 and column 2. The switch for 7 is connected from row 3 to column 1. The switch for 0 is connected from row 4 to column 2. The switch which is star on the phone is labeled down and the pound sign is labeled up. Each of the 4 rows has a 4.7 k ohm resistor going from it to plus 5 volts. each of the three columns is connected to the anode of a diode, 1N4148. The diodes are designated D1 for row 1, D2 for row2, and D3 for row 3. The cathode of D1 connects to pin 10 of the P I C 16C57C. The cathode of D2 connects to pin 11 and D3 to pin 12. Row 1 of the keypad array connects to pin 18 of the P I C 16C57C. Row 2 to 19, row 3 to 20 and row 4 to 21. pin 22 is the TX (transmit) / RX (receive) control pin. It is shown hard wired through a 4.7 k ohm resistor to plus 5 volts. Pin 23 goes to pin 5 of the PLL board plug. Pin 24 to pin 4 and pin 25 to 3. A ceramic resonator is connected between pins 26 and 27. It is a three terminal device. This device has a symbol similar to a quartz crystal except the center block, which usually represents the quartz plate, is shown as connected to ground. Pins 1, 2, 3, 4, and 7, of the 24LC018 are all tied to ground. Pin 8 goes to plus 5 volts. There is a 0.1 microfarad capacitor connected from plus 5 volts to ground. There is also a resistor network consisting of an 8.2 k ohm resistor in series with a 1.6 k ohm resistor connected from plus 5 to ground. The 1.6 k is closest to ground. The junction of these two resistors connects to pin 3 of the LCD. Pin 1 of the LCD is grounded and pin 2 goes to plus 5 volts. Pin 2 of the PLL board plug goes to ground and pin 1 to plus 5 volts. There is a socket shown mating with the PLL board plug. The socket has the following wire colors, 1 red, 2 black, 3 yellow, 4 green, and 5 white. These wires are labeled collectively to PLL board and disappear off the left edge of the diagram. End verbal description.
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