MIDI DIN Electrical Specification
The MIDI 1.0 Electrical Specification description and example circuit given in the original 1.0 document (1983) used 5 Volt electronics as was common at that time. The Specification was updated in 2014 to reflect current design requirements such as 3.3 Volt circuitry and reduced RF interference.
Electrical Specification Update 1.1 
This updates the MIDI 1.0 Electrical Specification to include 3.3-volt signaling. This update also adds optional ferrite bead RF filters to the signal pins, and optional grounding provisions for the grounding shield connectors on the MIDI jacks.
The original circuit diagram is shown below for reference (it is the most common circuit in use today, and is still acceptable). For more details, please see The Complete MIDI 1.0 Detailed Specification.
Figure 1: Original Electrical Specification Diagram
(Note: This diagram is superceded by MIDI 1.0 Electrical Specification Update )