The Studio 5 A serial port carries all the MIDI data originating from devices connected to the MIDI ports to your computer. The B serial port on the Studio 5 carries only data generated by the Studio 5 itself, i.e. the Studio 5's foot switch, foot controller, time code and audio in events. So MIDI notes are not distributed between the serial ports.
Sorry… you're confused. Don't feel bad, the description in the manual of how the S5 works is confusing, to say the least.
You quoted a paragraph from the S5 manual, but you left out part of it. The paragraph actually reads:
NOTE: When transmitting data in a dual port configuration, as shown in Figure 3.4, the Studio 5 distributes MIDI equitably between ports “A” and “B.”However, when the Studio 5 receives data, information from MIDI instruments enters port “A” while MIDI generated by the Studio 5 (time code, footswitch and foot controller info, and Audio In events) enters port “B.” Separating the incoming time code from incoming MIDI messages provides insures accurate synchronization.The S5, with its resident CPU, configures and re-configures itself in a number of ways, depending on how it's connected and whether or not there are multiple S5's present. There are numerous ways to configure a stack of S5's (and a zillion MIDI devices) to handle the largest possible MIDI network, and MIDI data flows in and out of the two ports in the most efficient way possible in all configurations.
The paragraph of note here is pointing out that: when there is a lot of continuous data because of MTC code, the S5 can send note data thru one port and continuous code thru the other, thus allowing for more timing accuracy without the notes suffering because of getting "bumped" by time code.
An important thing to remember is that the S5 typically communicates data to and from the Mac at 8X the MIDI rate and buffers it to be passed thru at the correct moment. It can do that because the RS-422 serial 230.4kbps data rate (aprox. 8X MIDI) is
much faster than the standard 31.25kbps MIDI rate and bi-directional so there is almost never a traffic problem there. If TWO are in use, that's just that much better.
USB 1.1, while NOT bi-directional, has a max speed of
12 Megabits, which is in another class entirely. Even having to send stuff in one direction at a time in serial order, when part of the MIDI system it's still by far the
fastest part and is never a factor in MIDI timing as long as the MIDI is properly buffered and timed on each end.
* The USB 1.0 spec was only 1.5Mbps but personally, I never saw an interface that actually operated at that speed.USB's only "fault" here is that it's a what-should-be-unnecessary conversion in the middle of the stream. It's in there only because Apple dumped the serial ports and
we want to use MIDI hardware and software that would rather have the serial ports present. In THAT sense, the SXPRO is a better way to go since it eliminates the need for the USB bus entirely and if more PCI-based serial port "restorers" actually existed and could be found, hardly anyone would use a little hang-on USB-to-serial converter box like the Keyspan… but:
The SXPRO is no faster than using the USB port via a Keyspan 28X and jitter is a non-issue in MIDI.
