MIDI INs and OUTs

The closest most people ever care to get to the heart of the MIDI interface are the three 5-pin ports found on the back of every MIDI unit. Labeled IN, OUT, and THRU, these ports control all of the information routing in a MIDI system The IN port accepts MIDI data, data coming "in" to the unit from an external source. This is the data that controls the sound generators of the synthesizer.

The OUT port sends MIDI data "out" to the rest of the MIDI setup. This data results from activity of the synthesizer, such as key presses, patch changes, and so on. The THRU port also sends data out to the MIDI system, but not in the same manner as the OUT port. The data coming from the THRU port is an exact copy of the data received at the synthesizer’s IN port. There is no change made to the data from the time it arrives at the IN port to the time is leaves the THRU port (which is a very, VERY small amount of time).

MIDI makes use of special five conductor cable to connect the synthesizer ports. Curiously though, only three of the conductors are actually used.

5 PIN Cable:
Data is carried through the cable on pins 1 and 3.
Pin 2 is shielded and connected to common.
Pins 4 and 5 remain unused.

Not just any cable will suffice for the exactness of the MIDI system, either. MIDI cable is specially grounded and shielded to ensure efficient data transmission. This means that MIDI cable is a little more expensive than standard 5-conductor cable, but reliable data transmission is absolutely necessary for MIDI.

The length of the cable is critical as well. IMA specifications suggest an absolute maximum cable length of 50 feet because of the method of data transmission through the cable.

The entire length of a MIDI chain is unlimited, however, provided that none of the links are longer than 50 feet. The optimal maximum length for cable is about 20 feet, and most commercially manufactured cable comes in five to ten foot lengths.

A MIDI chain describes a series of one-way connections in a MIDI setup. The elemental chain is a single-link chain. The MIDI OUT port of one device is connected to the MIDI IN port of a second. In this configuration, a key pressed on the first unit will cause both units to sound. Pressing a key on the second unit, however, only causes the second unit to sound.

Many instruments may be chained together using a series of single links to connect the units. In this case, the OUT of the first unit is connected to the second, the THRU of the second is connected to the IN of a third, and so on. If all the units are set to receive on the same channel, pressing a key on the first one will cause all the units to sound. Pressing a key on any of the other units will only activate the sound of that unit.

A MIDI loop is a special configuration of a MIDI chain. The single element loop is made of two interconnecting links. This was the configuration used in the debut of the MIDI system.

The OUT port of the first unit is connected to the IN port of the second, and the OUT port of the second is connected to the IN port of the first. In this case, as described earlier, a key pressed on either unit causes both units to sound, provided they are on the same channel.

A MIDI feedback loop does NOT exist here, as the data going into the second unit from the first is not duplicated in the OUT port of the second going back into the first. Here, we have two one-way links connected, not a multi-link chain.

Previous Lesson : More on MIDI Technology