Professional MIDI Guide
The MIDI 1.0 Specification also states:
The plug given as an example is a 180 degree (sometimes called 45 degree) type and not the 240 degree type. The cable is wired with the twisted pair using pins 4 and 5 and pins 1 and 3 are not used. The outer shell screen of the plugs must not be connected to the cable screen which should only use pin 2. This is because the socket shell on the equipment is normally connected to chassis ground and any unintended connection between different chassis is likely to cause ground hum and RFI problems. The screen 0V is provided by the MIDI Out and this should be preserved along the cable run without interruption or connection to other grounds. It is important to observe this through patching systems, do not connect all the cable screens together at a patchfield. Some equipment, particularly the type using an external mains adapter, may not be earthed and should not receive one via its MIDI Out connector.
There are several factors which limit the cable length, the two most significant are the MIDI Out hardware circuit itself and the opto-isolator used in the MIDI In circuit. Neither of these can be changed and the opto-isolator may also vary considerably. If a good cable such as those suitable for balanced audio or data transmission is used and the maximum length is not exceeded then reliable operation may be expected. Deviations from the standard may lead to all sorts of intermittent problems which inevitably lead to loss of work so don't risk it. The MIDI specification has carefully taken into account factors such as opto-isolator sensitivity, temperature variations and ageing. Straying from the standard cannot be guaranteed to work with all equipment combinations in all circumstances.
In general, any good quality microphone or data cable may be used, but it should be noted that the MIDI specification was drawn up over a decade before current EMC legislation was introduced. MIDI was designed to work well within a typical studio installation without introducing digital breakthrough or ground hum loops, but it was not designed with radio frequency interference in mind. As nearly all MIDI equipment contains microprocessors with clock frequencies above 1 MHz all connected cables have the potential to act as aerials regardless of the equipment meeting any regulations or not. This is because the equipment was only tested in a "typical" situation and if the manufacturer does not supply the cabling it does not have to be tested with cables attached.
Cables with "lapped" or "helical" screens should be avoided if possible as they have poor screening properties above the audio bandwidth. They may also cut through the insulation if flexed a lot or if stored knotted up. Use a braided screen cable for equipment connections and foil screened twin pairs for permanent installations. Ideally a foil screen with overall braid should be used, but these are quite hard to source. As no DIN connector used for MIDI is ever going to be suitable for RF screening this is always going to be a compromise.