Keithley SourceMeter 2425, Руководство по обслуживанию

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Model 2425 Service Manual Troubleshooting 4-13
sent back to the digital board through the RXB line via PD1. The 4MHz clock for the
microcontroller is generated on the digital board.
DS901 is the VFD (vacuum fluorescent display) module, which can display up to 49 char-
acters. 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).
The front panel keys (S901-S931) are organized into a row-column matrix to minimize the
number of microcontroller peripheral lines required to read the keyboard. A key is read by
strobing the columns and reading all rows for each strobed column. Key down data is
interpreted by the display microcontroller and sent back to the main microprocessor using
proprietary encoding schemes.
Troubleshooting
Troubleshooting information for the various circuits is summarized below.
Display board checks
If the front panel display tests indicate that there is a problem on the display board, use
Table 4-1. See “Principles of operation” for display circuit theory.
Table 4-1
Display board checks
Step Item/component Required condition Remarks
1
2
3
4
5
6
Front panel test
J1033
U902, pin 1
U902, pin 43
U902, pin32
U902, pin 33
Verify that all segments operate.
+5V, ±5%
Goes low briefly on power up, and then
goes high.
4MHz square wave.
Pulse train every 1 ms.
Brief pulse train when front panel key is
pressed.
Use front panel display test.
D5V supply.
Microcontroller RESET.
Controller 4MHz clock.
Control from main processor.
Key down data sent to main
processor.