Keithley 2015 Service Manual page 73

4-10 Troubleshooting
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).