Can a Dreamcast load CDI games from a IDE CDROM?

[Lynxman]

I'm considering making a handheld from one of my Dreamcasts. It's missing the GD-ROM as it was a parted out console I picked up cheap after someone's failed recap attempt. I was thinking of getting a slot in CD ROM drive and modifying it to work with that as a handheld but don't know if it can even read games that way. Hard drives work fine with DreamShell as slaves but what about a CD-ROM as the primary drive without running DreamShell? A GDEmu would be the obvious solution but it's not interesting enough. Off course a real GD-ROM drive with the disc spinning in free air sticking out of the console would be pretty fun too. What gyroscopic precession?

BTW this dreamcast works now after my repairs but the GPU seems to be slightly faulty (artifacting for a few minutes until it warms up, after which it works normally) which means I can mess around with it as much as I like without feeling like I'm doing something irreversible to a classic console.

 

[Y2K]

EDIT - See the below addendum. I did some further research, looks like Dreamshell might actually support doing this over G1.

Unfortunately this does not work. IDE CD-ROMs use fundamentally different communication protocols over the same "IDE" connection, and one that Dreamshell/whatever else isn't equipped to deal with. Hard drives use raw ATA, which is a protocol for doing direct block reads & writes to a hard drive, while CD-ROMs use ATAPI, and acts more like a packet device with a SCSI-like protocol. You may be able to get it working, but it depends if the hardware can support ATAPI at all, and you'd have to heavily modify Dreamshell.

Addendum: I got a bit curious and wanted to validate my claim, so I had an LLM look through the codebase and confirm. It turns out the codebase actually does sorta support ATAPI CD-ROM over the G1 bus (the GD-ROM connector)! I apologize for the misinformation, it seemed so impossible to me that I just assumed that had to not be the case. However, the functionality appears abandoned. The loader has a G1 driver that supports ATAPI and can parse ISO9660 disks (standard CD format) to find game images. That part seems fairly complete, however the shell never had specific support added, so some code modifications would be required even still. Regardless the intent was there, most of the plumbing was free with supporting GD-ROM as it's also an ATAPI drive, just with Sega's own command set, but there are explicit mentions of CD, and even DVD support!

 

[Lynxman]

Yeah I found several reports info about it being possible with Dreamshell without any LLM but noone seems to have reported what happens when they actually try except someone using a Blue-Ray drive, which wasn't going to work seamlessly anyway. I want to try reading the same kind of burned CD-rom disc that both the Dreamcast and my PC can read. I guess I'll just have to wire it up and try.

EDIT:

I think there's a clock signal on the GD-ROM PCB that the Dreamcast uses for things that needs to be replicated to work correctly too.

Seems that particular issue has a workaround posted here on BitBuit: https://bitbuilt.net/forums/threads/nolds-alternate-dreamcast-pinouts-workarounds.1822/

 

[Lynxman]

I wired up an IDE connector and LED to test. I wired it form the top of the motherboard to pull the wires up throught he BTB-Mezzanine-connector hole in the shielding. BTW something I discovered doing research... if anyone's wondering the connector that you find on a GD-ROm og GDEmu is called it's a "Molex 52602-0579": https://no.mouser.com/en/ProductDetail/Molex/52602-0579?qs=z57USrEQgt3HD9sMQhGRnQ==

The IDE cable works fine with a slave IDE harddrive through Dreamshell (but only if I load Dreamshell with a GD-ROM drive as loading DS with GDEmu didn't find the harddrive). I can browse and start images from the HDD. Nothing spescial so far, just confirming that the cable is working.

I wired the 33 MHz clock jumper wire to the bottom of the motherboard before the next step. Without it the console wouldn't play the startup animation and was unresponsive. I used a standard old working 5-1/4" IDE CD-ROM as Master without any other drive. It loaded into the CD audio playback menu when it booted with a game disc but no tracks were playing or available to select from. I didn't try with a standard audio CD. When I opened and closed the tray there was no message on screen about reading disc like there would normally be on the start page.

I also tried accessing the CD-ROM as slave with Dreamshell but The CD-ROM isn't detected even when loading Dreamshell from the GD-ROM drive (or a GDEmu though loading DS via GDEmu does't even find a slave HDD). I removed the clock jumper wire under the motherboard before this step.

Seems like I need a genuine GD-ROM drive to make a goofy handheld. You'd think they are cheap as everyone throws them away after installing a GDEmu but they are quite pricey from Ebay or Aliexpress.

This Dreamcast has the stock BIOS so I haven't tried loading directly to the HDD or CD-ROM.