Logic Pro 7: MIDI clock is always sent to all ports

Even if you choose a specific port for MIDI clock transmission (File > Song Settings > Synchronization... > MIDI), the MIDI clock signal is still sent to MIDI port 0 (all ports).

MIDI Clock is always sent globally to the MIDI interface which then distributes it to all ports. This works similarly with all multi-port MIDI interfaces. This method has the advantage that the MIDI clock signal is sent only once to the interface and not multiple times for each output port, thus reducing the MIDI data stream.

Depending on your MIDI interface, you may filter MIDI clock by filtering "realtime messages" in the setup software for the MIDI interface itself. Emagic MIDI interfaces allow this with the included Unitor Control software.