Digital Circuitry - Keithley 2700 Service Manual

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 require +37VDC 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
strobing the columns and reading all rows for each strobed column. Key-down data is inter-
preted by the display microcontroller and sent back to the main microprocessor using propri-
etary encoding schemes.

Digital circuitry

Refer to Figure 4-2
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 19-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 on power-up.
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 battery backed 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.
U171, Y103, and BT100 make up the battery watchdog control, along with the real-time
clock. U171 automatically senses when the +5VD supply is being powered down and then
switches to BT100 for power.
for the following discussion on digital circuitry.
Troubleshooting 4-7