MPU-401

The MPU-401, where MPU stands for MIDI Processing Unit, is an important but now obsolete interface for connecting MIDI-equipped electronic music hardware to personal computers. It was designed by Roland Corporation, which also co-authored the MIDI standard.

Released around 1984, the original MPU-401 was an external breakout box providing MIDI IN/MIDI OUT/MIDI THRU/TAPE IN/TAPE OUT/MIDI SYNC connectors, for use with a separately-sold interface card/cartridge ("MPU-401 interface kit") inserted into a computer system. For this setup, the following "interface kits" were made:

In 2014 hobbyists built clones of the MIF-IPC-A card for PCs.

Later, Roland would put most of the electronics originally found in the breakout box onto the interface card itself, thus reducing the size of the breakout box. Products released in this manner:

Still later, Roland would get rid of the breakout box completely and put all connectors on the back of the interface card itself. Products released in this manner:

By the late 1980s other manufacturers of PCBs developed intelligent MPU-401 clones. Some of these, like Voyetra, were equipped with Roland chips whereas others had retro-engineered ROMs (Midiman / Music Quest).

Examples:

In 2015 hobbyists developed a Music Quest PC MIDI Card 8BIT clone.

The MPU-401 can work in two modes, normal mode and UART mode. "Normal mode" would provide the host system with an 8-track sequencer, MIDI clock output, SYNC 24 signal output, Tape Sync and a metronome; as a result of these features, it is often called "intelligent mode". Compare this to UART mode, which reduces the MPU-401 to simply relaying in-/outcoming MIDI data bytes.

As computers became more powerful, the features offered in "intelligent mode" became obsolete, as implementing them in the host system's software became more efficient (than paying for dedicated hardware that will do them). As a result, the UART mode became the dominant mode of operation, with many clones not supporting the "intelligent mode" at all, being advertised as MPU-401 compatible.

In the mid 2010s a hobbyist platform software interface, SoftMPU, was written that upgrades UART (non intelligent) MPU-401 interfaces to intelligent MPU-401 interface.

In 2015 a PCB (HardMPU) was developed that incorporates SoftMPU as logic on hardware (so that the PC's CPU does not have to process intelligent MIDI).

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Wikipedia