Keithley 2701, Service Manual

Page: 89 / 168

4-12 Troubleshooting Model 2701 Service Manual
Ohms open-lead sense detection
There are two types of open sense lead detect ( Figure 4-4 ). The first is for the INPUT HI and
INPUT LO leads. For these leads, the open sense detection is implemented in hardware. A
comparator circuit monitors the voltage at the INPUT HI lead output and will trip the OVLD
detect circuit when the voltage level exceeds an appropriate value for the particular range. This
circuit operates because the INPUT HI lead output is connected to a current source. If the lead
is disconnected, it will quickly charge the output to the current source maximum voltage level.
This level depends on the range and is documented in the specifications in Appendix A. When
the open sense lead is detected, the front panel will display OVRFLW.
The second type of open sense lead detection is implemented in software. This function
works for the SENSE HI and SENSE LO leads. The SENSE HI and SENSE LO leads will drift
negative when disconnected due to small leakage currents. These leads, when operating prop-
erly, will always have a positive voltage on them; and when disconnected, they will drift to neg-
ative voltages. The open lead sense software monitors the voltage at these inputs and will trip
the open sense detect when the inputs drift to -15mV. When the open sense condition is
detected, the front panel will display OVRFLW.
Figure 4-4
Ohms open-lead sense detection block diagram
Scanner card signals
Scanner card input signals are connected directly to installed scanner cards. Scanner card
output signals are routed internally to the INPUTS switch, which selects between the front
panel terminals and the scanner card outputs.
DUT
SENSE HI
INPUT HI
INPUT LO
SENSE LO
Current
Source
Software Detect
Analog
Common
Hardware Detect
Software Detect