Cloudschatze wrote on 2023-10-18, 03:25:
The discrete GS program and wave ROMs are on the opposite, unphotographed side of the RLP daughterboard.
just to say - you're 100% right - I got my hands on a CM-300 unit and desolder the RLP board to see its other side. what was the surprise is that the RLP board was not "RLP-3037" (as on the pictures I took from Internet and posted in my initial post), but "RLP-3170".
So, it seems earlier CM-300 units (from 1991) were using "RLP-3037" with V2 GS Control ROM (I am assuming that based on earliest SCC-1 card, as I've never seen the other side of "RLP-3037") and later (1992) units are using "RLP-3170" with V3 GS Control ROM (that is confirmed based on the "RLP-3170" board that I took out).
Now, it's another story what is the actual difference between V2 and V3 GS Control ROM and if it really makes any difference for the end-user. Now, V4 GS Control ROM is SC55mkII and that is essentially no longer SC55, but V2 and V3 GS Control ROM, maybe are 100% SC55 compatible.
P.S. those V2, V3, V4 are not the actual version that is reported in software by the firmware, but what is written as markings on the chip itself.
[EDIT] BTW, those "RLP-3037" and "RLP-3170" boards look like good target for custom solutions, because I guess their pins are MIDI In/Out, Power and I2S signals, i.e. good target for example to take them out and build your own device with better DAC or SPDIF output.