Keithley 2461 Quick Start Manual page 14

You can use the Keithley Instruments connector CS-1616-3, supplied
with the Model 2461, to make the interlock connection to the rear
panel. You must supply connection wire.
To ensure proper interlock operation, the external interlock switch
and connection wires must be less than 10 Ω when the switch is
closed.
The pin locations and connections are shown in the following figure.
The pins are:
 Pin 3: Earth and chassis ground
 Pin 2: Interlock
 Pin 1: +6 V DC out (current limited)
For the examples shown in this quick start guide, you do not need to
use an interlock. The Model 2461 functions on all current ranges and
up to ±42 volts without asserting the interlock.