HP 8112A, Operating, Programming And Servicing Manual

Page: 276 / 406

After the appropriate control instruction, eight bytes of display data
are loaded by the microprocessor using eight successive WRITE
pulses.
HP-IB Status Latch U37
The data stored in the HP-IB status latch drives the HP-IB status
LEDs on the frontpanel.
Display Latche U23
The data stored in the display latch drives the parameter LEDs on
the frontpanel.
Reset Circuits
When the instrument is switched on, the microprocessor RESET
input is held low
(
reset
)
for approximately 2.5 ms. This allows the
power supplies to become established before the microprocessor starts
running. This delay is achieved using the CR network R12 (3
x
lOK)
and C4
(
0.1 µF
)
. U27D output goes high when C4 has charged to
approximately 420 m V and the RESET signal is withdrawn.
When the instrument is switched off, the "Power Down Detected"
signal goes high forcing the output of U27 A high. This switches on
transistor U29A which discharges C4 and switches the output of
U27D low. The RESET signal to the microprocessor and GPIA is
therefore established before their power supply is totally withdrawn.
1 0. 7 ·6 Servicing the Microprocessor and Front-panel