HP 8112A, Operating, Programming And Servicing Manual

Page: 275 / 406

Control Board Address
Decoding
Key Scanning
Display Driving
Address decoding for the control board functions is partially carried
out on the microprocessor board. Address lines A3-A9 and A14 are
passed through latch U18 under the control of sub-decoder 2 (U14).
A14 then becomes the MODE control for the display driver (refer
to "Display Driving" ) , while A3-A9 pass through bus driver U21.
Decoder U l 7 uses A6 and A 7 to produce the "write select" signals
WS1-WS6 and the WRITE signal to the display driver (refer to
"Display Driving") .
Refer t o Figure 10. 7-6 and Figure 1 0 . 7-7 The keyboard assembly
A4 is a switch panel on which all mode, control, parameter,
waveform, output and trigger control pushbuttons are mounted.
The microprocessor scans the frontpanel key matrix using a BCD to
decimal converter U20 and an 8-to-l multiplexer U19. A3, A4 and
A5 are the inputs to U20 and are continuously incremented from one
to six by the microprocessor. The six "decimal" outputs from U20,
KDO-KD5, are used as the vertical signal paths to the key matrix.
The horizontal signal paths of the key matrix, KSO-KS7, form the
inputs to the multiplexer U19, which is controlled by address lines
AO, Al and A2. The output from U19 therefore represents the state
of the key joining the vertical signal path (addressed by A3-A5) and
the horizontal signal path (addressed by AO-A2). Each time the
microprocessor increments the address to U20 it cycles the address to
Ul9 through all 8 horizontal paths.
Display Driver U22
The display driver operates the key, mode, control and unit LEDs,
along with the individual digital display segments, using a matrix
technique similar to the keyboard. The outputs DIGO-DIG7 form
the vertical signal paths while the outputs a-g and DP form the
horizontal paths. The display driver contains 8 bytes of RAM which
store 8 data bits (a-g and DP) to be used with each of the 8 "digit"
outputs DIGO-DIG7.
The MODE signal determines whether the display driver interprets
data as control instructions or display data to be stored in RAM:
Signal
M
ODE
WRITE
Table 1 0.7-1 . Display Driver Control Signals
Pin Status Function
9
8
HIGH Load control instruction on WRITE pulse
LOW Load display data on WRITE pulse
HIGH Data not loaded
LOW Data loaded
Servicing the Microprocessor and Front-panel 1o.7 -5