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.
RS-232 interface
Serial data transmission and reception is performed by the TXDB and RXDB lines of the
MPU. U159 provides the necessary voltage level conversion for the RS-232 interface port.
IEEE-488 interface
U158, U160 and U161 make up the IEEE-488 interface. U158, a 9914A GPIA, takes care of
routine bus overhead such as handshaking, while U160 and U161 provide the necessary buffer-
ing and drive capabilities.
Trigger circuits
Buffering for Trigger Link input and output is performed by U146. Trigger input and output
is controlled by the IRQ4 and PB3 lines of the MPU. U164 provides additional logic for the trig-
ger input to minimize MPU control overhead.
At the factory, trigger output is connected to line 1 of the Trigger Link connector (resistor
R267 installed). Trigger input is connected to line 2 of the Trigger Link connector (resistor R270
installed).
2-8 Troubleshooting
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