Keithley SourceMeter 3A 2420, Руководство по обслуживанию

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Troubleshooting 4-9
Display board circuit theory
Display board components are shown in the digital circuitry block diagram in Figure 4-5.
U902 is the display microcontroller that controls the VFD (vacuum fluorescent display) and
interprets key data. The microcontroller has four peripheral I/O ports that are used for the various
control and read functions.
Display data is serially transmitted to the microcontroller from the digital board via the TXB
line to the microcontroller PD0 terminal. In a similar manner, key data is serially sent back to
the digital board through the RXB line via PD1. The 4MHz clock for the microcontroller is gen-
erated on the digital board.
DS901 is the VFD (vacuum fluorescent display) module, which can display up to 49 charac-
ters. Each character is organized as a 5
×
7 matrix of dots or pixels and includes a long under-
bar segment to act as a cursor.
The display uses a common multiplexing scheme with each character refreshed in sequence.
U903 and U904 are the grid drivers, and U901 and U905 are the dot drivers. Note that dot driver
and grid driver data is serially transmitted from the microcontroller (PD3 and PC1).
A/D
Control/Data
Voltage Source
Control
ROM
U15, U16
RAM
U12, U14
Reset
E PROM
U17
2
A/D Interface
U9, U25
Microprocessor
U3
Serial
Interface
U4
GPIB
U6, U13
U20
RS-232 Interface
IEEE-488 Interface
To Display
Board Controller
Trigger
U23
Digital I/O
U7
Trigger
Digital
I/O
16.78MHz
Figure 4-5
Digital circuitry
block diagram