Keithley SourceMeter 2400, Service Manual, Page: 62 / 137

2400-902-01 (G - Feb 2006)
Model 2400 Service Manual 4-8
Troubleshooting
A/D converter
The SourceMeter unit uses a multi-slope charge balance A/D converter with a single-slope run-
down. The converter is controlled by gate array U610. Commands are issued by the MPU on
the digital board through communications opto-isolators to U610, and U610 sends A/D reading
data back through opto-isolators to the digital board for calibration and processing.
Active guard
The Model 2400 has an active guard or “six-wire ohms” circuit used to measure complex
devices. This circuitry provides a low-current (50mA) equivalent of the voltage on output HI. If
the unit is in the SV mode, the low-current equivalent of the source voltage will appear on the
guard terminal. If the unit is in the SI mode, the voltage on output HI is equal to the source cur-
rent multiplied by the external resistance value. An equivalent voltage will be generated by the
guard circuit, and a guard sense terminal is provided to sense around the voltage drop in the
guard leads since significant current can flow (50mA).
Digital circuitry
Refer to Figure 4-4 for the following discussion on digital circuitry.
The core digital circuitry uses a Motorola 68332 microcontroller running at 16.78MHz. The
memory configuration includes two 256K
×
8-bit EEPROMS and two 128K
×
8-bit RAMs
used in parallel to utilize the 16-bit data bus of the MPU. The RAM is battery backed-up, pro-
viding continued storage of data buffer information during power-down cycles. All calibration
constants and system setups are stored in a separate serial EEPROM.
External communication is provided via GPIB and serial interfaces. A 9914 GPIA IEEE-488
standard interface IC is used for the GPIB, and a 68332 Queued Serial Module (QSM) provides
the serial UART. For internal communications, the Time Processing Unit (TPU) is used for
serial communications with the front panel display module, and both the TPU and QSM handle
digital-to-analog interfacing.
Display board
Display board components are shown in the digital circuitry block diagram in Figure 4-4.
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 vari-
ous 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
generated on the digital board.