Half rack unit, additional height for more controls, introduced multiple triggering and EQ, SC-55 map support (with differences).
Table top version of SC-50, rebranded as BOSS Dr.
SC-55mkII without the Roland MT-32 patches, only one MIDI input. Piano module, with a limited set of instruments. No display, introduced serial port for PC/Mac connectivity.
Half Rack unit, first product of the line, units without GM logo technically not GM compatible (GM reset interpreted as GS reset, capital tone arrangement not fully consistent with GM spec). In some cases also sold as 'Edirol' rather than 'Roland' as the brand name. 1.2 Sound Canvas Personal Computer Products.