XLR 4-Pin wiring for DC power connections
There is no universal standard for this, however the most common convention for DC power on XLR 4-pin connectors is:
Pin 1 engages before the other pins when mating, so is ideally the ground contact. This arrangement is used on all relevant Canford manufactured products.
Another well used convention can be found on many cameras, where pin 1 is 0V and pin 4 +12V, others not connected. A further specialized use of the XLR 4-pin when DC power is carried is the DHD RM4200D CAN bus connector, which has pin 1 at +24V and pin 4 (connected to shell) at 0V. The bus signal is on pins 2 and 3. This arrangement is to apply power before the bus, to allow for hot plugging.
A BBC Radio Outside Broadcasts convention, now virtually obsolete, was to have pin 1 as 0V or negative, pin 2 as +12V relative to pin 1, pin 3 +24V relative to pin 1, and pin 4 +48V relative to pin 1. Thus a -50V supply would be on pin 1 and pin 4 would be ground (and pins 2 and 3 would not be used).
This is essentially the same as the common convention, except that the grounded pin effectively ‘floats’. Unless there were common earths with other equipment, any equipment wired to one convention would pick up the correct voltages, if available, at the correct polarity from a PSU wired to the other.