Keithley 2015, Service Manual

Page: 72 / 158

Troubleshooting 4-9
Display
DS401 is the display module, which can display up to 12 alpha-numeric characters and
includes the various annunciators.
The display uses a common multiplexing scheme with each character refreshed in sequence.
U402 and U403 are the drivers for the display characters and annunciators. Note that data for
the drivers are serially transmitted from the microcontroller (MOSI and PC1).
Filament voltage for the display is derived from the power supply transformer (F1 and F2).
The display drivers r37VDC and +5VDC, which are supplied by U144 (+5VD) and
U101 (+37V).
Key matrix
The front panel keys (S401-S430) 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 strob-
ing 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 encod-
ing schemes.
Digital circuitry
Refer to Figure 4-2 for the following discussion on digital circuitry.
Microprocessor
U135 is a 68306 microprocessor that oversees all operating aspects of the instrument. The
MPU has a 16-bit data bus and provides an 18-bit address bus. It also has parallel and serial ports
for controlling various circuits. For example, the RXDA, TXDA, RXDB and TXDB lines are
used for the RS-232 interface.
The MPU clock frequency of 14.7456MHz is controlled by crystal Y101. MPU RESET is
performed momentarily (through C241) on power-up by the +5VD power supply.
Memory circuits
ROMs U156 and U157 store the firmware code for instrument operation. U157 stores the D0-
D7 bits of each data word, and U156 stores the D8-D15 bits.
RAMs U151 and U152 provide temporary operating storage. U152 stores the D0-D7 bits of
each data word, and U151 stores the D8-D15 bits.
Semi-permanent storage facilities include NVRAM U136. This IC stores such information as
instrument setup and calibration constants. Data transmission from this device is done in a serial
fashion.