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4.2.5 Keypad and Display (Refer to Schematics #72082 and #72080 or #72115)
The keypad switches are arranged in a 4 by 4 X-Y grid. Address lines A0 to A3 drive the grid rows through
74LS367 buffer U10 (U12 in the DP7) and the grid columns are read onto the 6809 data bus through
74LS245 transceiver U19 (U22).
The display is a 2 line by 16 character liquid crystal display (LCD) with integral electronics. All data transfers
between the 6809 and the display are buffered through transceiver U19 (U22). When the 6809 is ready to
display data, it reads status from the LCD and if the LCD is ready to accept data, the 6809 writes ASCII
characters to the LCD. The LCD then generates and displays the corresponding character patterns.
4.2.6 Sensors
Four optical sensors are used: the left home sensor for carriage position, the front and rear document sensors
for document presence and position and the sensor on the keypad PCB for cover open condition. All sensors
operate in the same way: a constant current is provided to the integral light emitting diode (LED) of the sensor
through a resistor to +5 volts. The LED light turns on the corresponding phototransistor, raising its emitter
voltage to about 3.5 VDC. When an object is placed in the slot between the LED and the phototransistor, the
transistor turns off and its emitter is pulled down to ground by a resistor. Sensor status is read by the 6809 via
74LS365 buffer U7 (U8 in the DP7). Printers with the auto alignment feature add a fifth sensor for detecting
skew of the inserted document. Both this sensor and the rear document sensor are reflective sensors which
turn on their phototransistors when a document is present.
4.2.7 Character Printing
The carriage motor steps the carriage in 1/120" increments but dots can be printed twice per increment (3
times per increment in the DP7). Printing at 10 characters per inch (cpi) uses a 9 wide by 8 high (13 X 22)
character matrix. Nine (13) increments are used for printing a character and 3 (5) increments for the
intercharacter space. Printing at 12 cpi uses a 7 X 8 (11 X 22) matrix with 7 (11) increments used for printing
and 3 (4) for spacing. Printing at 17.1 cpi uses a 9 X 8 (7 X 22) matrix with 9 half (7) increments used for
printing and 5 (3.5) for spacing. At correspondence quality (CQ) speed half increment printing is used to
double the horizontal resolution and the matrix doubles to 17 X 17 (25 X 22) for 10 cpi and 13 X 17 (19 X 22)
for 12 cpi. DP6 CQ is printed in two passes with the document moved by 1/2 dot between passes to double
the vertical resolution. DP7 CQ is printed in a single pass.
At all print speeds only every other dot position can be printed so the maximum number of horizontal dots
printed is 4 for a 7 wide matrix, 5 for a 9 wide, 6 for an 11 wide, 7 for a 13 wide, 9 for a 17 wide, 10 for a 19
wide and 13 for a 25 wide matrix. In all print modes the upper 7 (17) printwires are used for printing normal
characters, the eighth (18th through 22nd) wire for lower case descenders and the ninth (23rd & 24th) wire for
underlining.
Printwire impact force and print density decrease as the printwire cycling frequency increases. Draft and CQ
print at a maximum frequency of 1,000Hz (1,350Hz) and normal print occurs at a maximum frequency of
750Hz (1,350Hz).
