Keyboard/Display Control; Troubleshooting Modes; Guard-Crossing Communication; Guard Crossing - Fluke 8842A Instruction Manual

8842A
Instruction Manual

5-32. Keyboard/Display Control

Keyboard/Display Controller U212 communicates with the In-Guard µC over the internal
bus. During a µC write cycle, address line A0 tells U212 whether to consider data being
sent by the µC as configuration commands or as display data. Display data is stored in the
Keyboard/Display Controller, which automatically scans the display. The
Keyboard/Display Controller selects one of eight grids using decoder U213 and buffer
U215. The numeric display data is decoded from BCD to 7-segment by decoder U216
and buffered by U217. Additional annunciator data is buffered by U218.
The Keyboard/Display Controller is reset by the µC whenever the µC is reset. It receives
a 1-MHz clock signal from the custom A/D IC (U101), which uses the µC 8-MHz crystal
for its clock input.
The Keyboard/Display Controller scans the keyboard, sensing pressed buttons on lines
RL0-RL7. It sends an interrupt to the µC via line KEYINT whenever a front panel button
is pressed. The µC then reads the keycode from the Keyboard/Display Controller. (The
status of the FRONT/REAR switch is sensed separately by line F/R SENSE.)

5-33. Troubleshooting Modes

In addition to running the diagnostic self-tests, the In-Guard µC has a troubleshooting
mode which aids in finding digital hardware problems. After the uC is reset, it senses the
relay control lines (U202-35 through U202-38) as inputs. If line U202-38 (TP205) is
shorted to ground, the µC goes into the troubleshooting mode. (U201 provides internal
pull-up.) The troubleshooting mode is described in detail in the Maintenance section.

5-34. Guard-Crossing Communication

The In-Guard µC contains a UART (universal asynchronous receiver transmitter) which
it uses to communicate across the guard to the IEEE-488 Interface. The transmission
speed is 62,500 bits per second.

5-35. GUARD CROSSING

The Guard Crossing consists of two identical circuits, each of which transmits data in one
direction across the guard isolation between the Main Printed Circuit Assembly and the
IEEE-488 Interface. One circuit is shown in Figure 5-15; the other circuit works
identically. A portion of each circuit is contained in the IEEE-488 Interface.
5-24
Figure 5-15. Guard Crossing Circuit
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