A MIDI thru port provides a copy of the MIDI signal arriving at a corresponding MIDI in port. The data from the MIDI THRU socket can then go forward to the MIDI IN socket of another receiving device. Some devices, especially older drum machines, do not have a MIDI thru connection; a device without a MIDI thru connector must be placed at the end of a MIDI chain. To make life easier, there are simple hardware gadgets called MIDI Thru Units or splitters.
Hardware and software sequencers tend to use the name thru for the feature where messages received at the MIDI in port are copied. to the MIDI out port.
Software elements involved with routing MIDI data in computers are sometimes called MIDI thru utilities. For Windows 95, 98, ME, NT and XP you may be interested in the versatile MIDI-YOKE drivers we use these. Hubi's MIDI Loopback device is a popular alternative, but at this time it does not install under XP (you can, however, still run the file HWMDCABL.EXE directly). The MIDI Thruway option from Techno Toys is unfortunately no longer available.
For use on the Mac, the OMS and FreeMIDI MIDI driver/management systems have traditionally fulfilled the same role. Mac OS X integrates MIDI services into the operating system, as well as an application called Audio MIDI Setup.
Because MIDI connections use a type of current loop, a single MIDI (THRU or OUT) output port can only reliably drive a single MIDI (IN) input port - so, you should not gang MIDI inputs in parallel or series. As a result, interconnecting several items of MIDI gear can be a frustrating business. A midi thru box, sometimes called a MIDI 'splitter', can often help - by letting one MIDI output talk to several MIDI inputs.
Some of your MIDI gadgets may lack the midi thru sockets (see above) which are required for connecting several devices together in a 'chain'. Where midi thru ports are provided, the signal throughput may introduce 'duty cycle' distortion or (more rarely) time delays. Consequently, 'chain' performance may be unacceptable - particularly on 'chains' of three or more. Also, all devices up the chain must be be powered-up to keep the chain working.
The preferred alternative to the troublesome 'chain' is the 'star' network. The only snag is that for a 'star' network you will need more than the single MIDI Out, which is all that may be available on your master controller. You'll need a midi thru box to solve this problem by setting up a MIDI 'star' system.
A MIDI network may be expanded via a hybrid combination of midi thru box 'star', and chained thrus.
We used to make a range of MIDI Thru Units
How to wire a DIN plug for MIDI
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