R-BUS cables have been out of production for years and we no longer have resources to purchase these. The best place to still find one would be online (For example: Searching on Google for "R-Bus cable", "Roland R-Bus cable", "Roland RBC-5", etc). Note that the R-BUS cable was a proprietary design and a SCSI cable/connector is not compatible.
It's recommended that cable lengths are no longer than 5 meters (16 feet).
R-BUS cables used a DB-25 connector with a crossover connection; pin 1 to 13, pin 2 to 12, pin 3 to 11, pin 4 to 10 etc. Pin 7 has no connection.
NOTE: Because of the proprietary design, we have not been given any additional design specs for R-BUS cables from our corporate headquarters from Roland Japan--as these are not published and are not available.