Sets whether constant voltage will be controlled externally.
|Value||NONE||External control is not performed (default).|
|VOLTage||Controls the voltage using an external voltage|
|RESistance||Controls the voltage using an external resistance|
For the setting that is applied when *RST is sent, see this table.
Returns whether constant voltage is being controlled externally in CHAR format in response to VOLT:EXT:SOUR?.