The SFG-05 / SFG-01 FM-units are built around the YM2151 / YM2164 chip. These IC’s have 4 operators and sound quality is comparable to the cheaper Yamaha DX series (e.g. the DX-9, FB-01), several arcade games and the Korg(!) DS-8 and 707.
To program this chip, there are two memory locations available (register select and data) see the Music function I/O map for details.
PDF Documents (Requires Adobe Acrobat reader):
CX5M-I Service Manual (funet.fi, PDF, 3.1MB)
CX5M-II Service Manual (scribd, PDF, 3.8MB)
YK-01 Service Manual (hansO, PDF, 145KB)
CX5M Owners Manual (hansO, PDF, 3.4MB)
CX5M-II Owners Manual (hansO, PDF, 1.5MB)
YM2151 Datasheet (ionpool.net, PDF, 246KB)
The 4-operator FM from the SFG modules
SFG-05 Music BIOS manual (hansO, PDF, 15.8MB)
YM2151 Register description (PDF, 1.3MB)
A detailed document about programming the YM2151
1793-02P Datasheet (PDF, 716KB)
The disk controller used in the FD-051 Floppydisk interface module.
FD-03 Service Manual (hansO, PDF 11.7MB)
The Yamaha Floppy Drive service manual
I don’t have information about the YM2148 which handles the MIDI and Keyboard scan but my guess is that it works the same way as embedded in the later YM8950 IC
Music function I/O map (HTML)
How to access the SFG from the computer
Programming the YM2151 (by NYYRIKKI )
HTML document of the YM2151 datasheet (with ‘find-routine’) Plain text document is here
SFG-05/01 to MSX Cartridge adapter
Originally sold as the UCN-01 (Yamaha Unit Connector)