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.
Design
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 inserted into a computer system. For this setup, the following "interface kits" were made:
MIF-IPC: For the IBM PC/IBM XT. It turned out not to work reliably with 286 and faster processors. Early versions of the actual PCB had IF-MIDI/IBM as a silk screen.
MIF-IPC-A: For the IBM AT, works with PC and XT as well.
Xanadu MUSICOM IFM-PC: For the IBM PC / IBM XT / IBM AT. This was a third party MIDI card, incorporating the MIF-IPC and additional functionality that was coupled with the OEM Roland MPU-401 BOB. It also had a mini audio jack on the PCB.
In 2014 hobbyists built clones of the MIF-IPC-A card for PCs.
Variants
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:
MPU-401N: an external interface, specifically designed for use with the NEC PC-98 series notebook computers. This breakout-box unit features a special COMPUTER IN port for direct connection to the computer's 110-pin expansion bus. METRONOME OUT connector was added. Released in Japan only.
MPU-IPC: for the IBM PC/IBM XT/IBM AT and compatibles. It had a 25-pin female connector for the breakout box, even though only nine pins were used, and only seven were functionally different: both 5V and ground use two pins each.
MPU-IPC-T: for the IBM PC/IBM XT/IBM AT and compatibles. The MIDI SYNC connector was removed from this Taiwanese-manufactured model, and the previously hardcoded I/O address and IRQ could be set to different values with jumpers. The break-out box has three DIN connectors for MIDI plus three 3.5mm mini jack connectors.
MPU-IMC: for the IBM PS/2's Micro Channel Architecture bus. In earlier models both I/O address and IRQ were hardcoded to IRQ 2 ; in later models the IRQ could be set with a jumper. It had a 9-pin female connector for the breakout box.. Due to the incompatibility of IRQ 2/9 between the MPU-IMC and IBM PS/2 MCA models certain games will not work with MPU-401.
S-MPU/AT : for the IBM AT and compatibles. It had a Mini-DIN female connector for the breakout box. The MIDI SYNC, TAPE IN, TAPE OUT, METRONOME OUT connectors was removed, but a second MIDI IN connector was added. An application to assign resources must be run to use the card in DOS. This application is not a TSR, i.e. it does not take up conventional memory.
S-MPU-IIAT : for the IBM or compatible Plug and Play PC's. It had a Mini-DIN female connector for the breakout box with two MIDI In connectors and two MIDI Out connectors. An application to assign resources must be run to use the card in DOS. This application is not a TSR, i.e. it does not take up precious conventional memory.
LAPC-I: for the IBM PC and compatibles. Includes the Roland CM-32L sound source. A breakout box for this card, the MCB-1, was sold separately.
LAPC-N: for the NEC PC-98. Includes the Roland CM-32LN sound source. A breakout box for this card, the MCB-2, was sold separately.
RAP-10: for the IBM AT and compatibles. General midi sound source only. MPU-401 UART mode only. A breakout box for this card, the MCB-10, was sold separately.
SCP-55: for the IBM and compatible laptops. Includes the Roland SC-55 sound source. A breakout box for this card, the MCB-3, was sold separately. MPU-401 UART mode only.
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:
MPU-APL: for the Apple II series. Single-card combination of the MIF-APL interface and MPU-401, featuring MIDI IN, OUT, and SYNC connectors.
MPU-401AT: for IBM AT and "100% compatibles". Includes a connector for Wavetable daughterboards.
MPU-PC98: for the NEC PC-98.
MPU-PC98II: for the NEC PC-98.
S-MPU/PC : for the NEC PC-98.
S-MPU/2N : for the NEC PC-98.
SCC-1: for the IBM PC and compatibles. Includes the Roland SC-55 sound source.
GPPC-N & GPPC-NA: for the NEC PC-98. Includes the Roland SC-55 sound source.
Clones
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. Examples:
In 2015 hobbyists developed a Music Quest PC MIDI Card 8BIT clone. In 2017/2018 hobbyists developed a revision of the Music Quest PC MIDI Card 8BIT clone that includes a wavetable header in analogy of the Roland MPU-401AT.
Modes
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. 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.
SoftMPU
In the mid 2010s a hobbyist platform software interface, SoftMPU, was written that upgrades UART MPU-401 interfaces to an intelligent MPU-401 interface, however this only works for the DOS operating system.
HardMPU
In 2015 a PCB was developed that incorporates SoftMPU as logic on hardware. Currently HardMPU only supports playback and not recording.
Contemporary interfaces
Physical MIDI connections are increasingly replaced with the USB interface, and a USB to MIDI converter in order to drive musical peripherals which do not yet have their own USB ports. Often, peripherals are able to accept MIDI input through USB and route it to the traditional DIN connectors. While MPU-401 support is no longer included in Windows Vista, a driver is available on Windows Update. As of 2011 the interface was still supported by Linux and Mac OS X.
