Help with new BBA adapter (cont. from Cluster)

[Chaotis]

Several people have made it known that they want a new bba adapter that doesn't cost 180$ or so.

I was hoping to start this topic where we can post any and all info on the subject and maybe eventually work towards building a new one. Anyone with information on the parallel port, programming for it, the current BBAs, hardware that might be usable, etc; please post it here.

This topic branched off the DC Cluster topic so you can also look there for more info.

[semicolo]

There's an error in the pictures I gave you, the guy who scanned them scanned his modem in place of hi BBA by mistake :-S

[wraggster]

a homebrew BBA would be a massive step for the scene

good luck

[Chaotis]

Guess I can start off with what we know already.

The BBA, or at least one model, uses the realtec 8139 ethernet contoller IC. This lets the dreamcast connect to an ethernet network.

The dreamcast parallel port has 16 data pins (pinout can be found on the internet) and operates at 25mhz. Allowing 50MB/s data transfer.

Most ethernet contollers (pci) operate with 32 pins at 33mhz.

I can think of 3 ways we may be able to connect the two together:
-Find a chip that can operate at 12.5mhz (half the parallel port) and send two sets of 16bits for each clock.
-Find an ethernet controller that uses 16bits already.
-Overclock the maple bus to be able to operate at twice the slowest pci clock. I've seen 16.75 so that means overclocking the bus from 25 to 35mhz. Then doing the pairs of 16bits idea.

Any ideas on feasability or know of something we can use?

[semicolo]

Should be easier to have a chip working at 25Mhz just translating 16bits in 32 with some buffers, the realtek must be working at 25Mhz (as one can change pci speed in one's pc bios and it won't affect ethernet cards).

Finding an ethernet chip working at 16 bits should lead us to obsolete parts, but can be done.

Overclocking the G2 bus (maple is for joysticks) doesn't seems a good idea for me, it could alter the standard behaviour of the DC since the sound chip is connected on it.

[Chaotis]

But wouldn't taking the 16bit to 32bit mean halving the mhz to 12.5? I haven't been able to find any controllers that will run at this speed. Or is there another way to do this? I'm taking classes on electronics but haven't gotten very far yet, maybe in the following years...

[semicolo]

It would mean buffering pci strobes of data.
Like i said, pci devices sould be able to run at 25Mhz.
If for example the pci device sends 16 times 32 bits (at 25Mhz), one keeps it in a memory and then (or even in the same time) one sends it on the g2 bus 32 times 16 bits.
In the other direction it would just be wait for two times 16 bits before writing 32 bits on the pci device.

That seems to be another job for SUPER CPLD defender of the logic and numerical data.

[pboese]

Sounds like an amibitious project ;D Semicolo's DC modem disassembly tutorial and extender board is a great start though http://semicolo.free.fr/Dreamcast/. Thanks for that semicolo!

[Chaotis]

Ah, I understand now. I thought it was something that was always streaming data. Not sure if that makes things easier or harder...

Other than telling the hardware when to wait and storing the seperate 16bit sections both ways, anything else that might cause problems between the two interfaces?

[quzar]

why not use an isa ethernet card as your basis? that way you dont need to do any weird things since it would be a 16 bit bus anyways...