HP 220 - Manual (Page 63)

5Ć31 Functional Overview C3187Ć90000
Front Panel PCA
The plotters use the same frontĆpanel PCA as the DeskJet 550. The frontĆpanel consists of
nine LEDs and eight switches, mounted on the PCA, which is deeply recessed behind a plastic
housing containing the key pad. The keys act on the switches through push rods. Light pipes
transmit the light from the LEDs to the display area.
LEDs
The LEDs are driven by drivers on the main board, under the control of the servo processor.
The LED layout is shown below:
led[1] led[4] led[7]
led[2] led[5] led[8]
led[3] led[6] led[9]
Front-Panel LEDs
Key Pad
The keypad layout is shown below:
led[2] led[3] led[6] led[7]
key[0]
key[1]
Switch[1] Switch[2] Switch[3] Switch[4]
Switch[5] Switch[6] Switch[7] Switch[8]
Front-Panel Key Pad
The switches are connected as a 4x2 matrix. The switch matrix, when scanned, is driven by
the four LED control signals led[2], led[3], led[6] and led[7]. The servo processor scans the
switch matrix by setting, in turn, each of the four control signals to `0' while holding the
other three at `1'. While driving the switch matrix, the processor reads the signals key[0],
and key[1]. A `1' on a key signal indicates a closed switch.