Keithley 2450 Reference Manual page 210

Model 2450 Interactive SourceMeter® Instrument Reference Manual Section 3: Functions and features
Vext line
The digital I/O allows connection to an external voltage through a flyback diode clamped line (Vext).
You can connect to the Vext line through the Vext pin (pin 5) and the specified digital I/O line. Use
this connection to drive relay drive coils, a low power solenoid, or similar external inductive circuitry.
The externally supplied voltage can be up to +5 V.
Do not apply more than 50 mA (maximum current) or exceed +5 V (maximum voltage) on the digital
I/O port. Applying current or voltage exceeding these limits may damage the instrument.
Refer to the following figure for a simplified schematic of a sample digital I/O Vext relay circuit. You
can externally power other devices by replacing the relay coil with the other device. When using the
Vext pin to control externally powered devices, make sure to configure the corresponding digital
output lines. In the low state (0 V), the output transistor sinks current through the external device. In
the high state, the output transistor is off (transistor switch is open). This interrupts current flow
through the external device. Most applications use an active-low (ON = 0 V) output sense.
Figure 93: Digital IO Vext (example external circuit)
+5 V line
The digital I/O port provides a +5 V output. You can use this line to drive external logic circuitry. The
maximum current output for this line is 500 mA. A self-resetting fuse with a one-hour recovery time
protects this line.
2450-901-01 Rev. B/September 2013 3-65