Apple Logic Pro X - Manual (Page 122)

Chapter 5 Connect external devices 122
m If your MIDI interface has only one MIDI output: You need to connect the MIDI In of the second
tone generator to the keyboard MIDI Thru port. A third device can be connected to the MIDI Thru
of the second unit, and so on.
Keyboard
In port Out port
In port Out port
MIDI interface
Computer
Tone generator
Tone generator
MIDI Thru port
MIDI Thru port
In port
In port
The MIDI Thru port replicates the signals coming into the MIDI In port of the device. It’s better to
use a direct connection from the computer MIDI Out port to a device, rather than chaining too
many units, one after the other. Doing so can cause timing problems in the chain if numerous
MIDI commands are sent quickly, due to the slight delays introduced by each MIDI In to MIDI
Thru transaction. As such, a multi input/output MIDI interface is recommended in studios with
several MIDI tone generators and controllers.