Why is INH pulled low to disable the 66xx power supply or 68xx ac source?

It might appear more logical to have a contact opening represent the disabled state in a "fail-safe" manner than contact closure. That way, the source would go into the disabled state in the event that the connector falls off or a wire breaks, or a micro-switch opens. However, the disadvantage of this approach is that it complicates communication between multiple supplies in a system.

Pulling the INH line low to disable the source makes it possible to "wire-or" together the FLT (fault) output signals for a group of power supplies and ac sources. Thus, an INH input signal (remote inhibit) to any of the units can be used to shut down all others in the system. Internal circuitry latches the INH line low until, keeping all supplies disabled until they are deliberately reset.

An additional reason for choosing the low state to disable the output is that it simplifies normal operation for users who do not need to use the disable capability. Thus, those users who are not using this capability do not have to remember to connect the required jumper wire, or plug in a connector with the shorting jumper. Otherwise, where a positive connection is required to establish normal operation, users frequently mistakenly conclude that the unit has failed, and that the output is dead.