Professional MIDI Guide
MIDI Long Haul
The MIDI Output circuitry was designed to withstand being plugged into another without damage, but this compromised the drive capability to 15 metres of normal screened twisted pair cable. It is possible to extend this slightly by using special low capacitance and resistance cable, but as the limitations are caused by the 660 ohms of resistors in the MIDI current loop only a few more metres may be obtained.
Even a small 5 x 4 metre room can easily use up the 15 metre limit when the cables are installed in ducting and connecting equipment between rooms or from a stage to the wings or FOH mixer can requires runs of 100 metres or more. Various techniques have been employed to extend the the MIDI limit. Analogue line drivers and receivers can transmit the distance, but have bandwidth and phase distortion problems which can corrupt the data. Conversion to RS422 has also been used, but this can cause ground loop and breakthrough problems in the audio equipment.
Naturally, we can only recommend our own Long Haul Converter system which has been designed to overcome all these problems and still provide full optical isolation while using standard XLR balanced tie lines or microphone cables.