The Apple MIDI Interface has one Output and one Input port. However, with these ports the interface can communicate with 16 differerent MIDI channels, which is the standard established by the MIDI Developers Association.
All 16 channels can be used by one MIDI device, such as a keyboard synthesizer, or the channels can be split up among several different MIDI devices. You could even have 16 different devices each playing one part of a song, each part being received on 1 channel. A common practice is for channel 10 to go to the device that is going to play the drum parts and then the other 15 channels are used to go to other MIDI devices. One MIDI device might play the Piano sounds while another one might have that awesome bass sound that you gotta have.
The Apple MIDI Interface only has one Input so it can only accept input from one MIDI device at a time. To receive input from two sources at the same time using the Apple MIDI Interface, you would need to add a MIDI merge box (from a third-party developer).
More complicated MIDI interfaces are available from third-party companies that offer multiple Input and Output ports. If you have a more involved setup that includes multiple input devices, such as keyboards, drum triggers, MIDI guitars, and so on, you should consider a more complicated MIDI interface. The multiple Outputs allows the musician to access 16 channels per output. If the MIDI interface has 8 outputs then data can be sent to 8x16 = 128 different MIDI channels.
This article was published in the Information Alley on 24 September 1996.