MIDI
The original DX7 included MIDI ports, but was released shortly before the specification was completed. As a result, its MIDI implementation is quite modest - it only transmits information on MIDI channel 1, it can receive information on any one of the sixteen MIDI channels at a time, but lacks the OMNI feature that enabled later DXs in the series to receive on all MIDI channels simultaneously. Very early DX7s manufactured in 1983 were distinctive for not having "MIDI Channel" inscribed next to the button that opens this function (button 8). This missing label was corrected early in the production run.
Additionally, the maximum MIDI velocity value that the DX7 will transmit is 100 (of the 127 maximum value defined by the standard). The DX7 will however, respond to the full range of velocity values when sent from an outside MIDI source. This means that when using the original DX7 as a MIDI controller to play external sound modules, the patches on these modules must be adjusted to be more sensitive to velocity. It also means that when playing the DX7's own sounds using an external MIDI controller or sequencer, the velocity values should be rescaled before the input to the DX7, or the DX7 patches would need to be adjusted to be less sensitive to velocity.
DX7 Mark II models resolved these deficiencies and were equipped with full MIDI implementation, including the ability to send and receive on any or all of 16 channels, send program change commands, controllers commands (total of 103 parameters can be assigned to any controller), and SysEx (system exclusive) messages, thus making them quite useful as a MIDI controllers for other synths and sound modules.
Read more about this topic: Yamaha DX7