M1565 External Drive Controller Board Design

As part of the work on the MEGA65 expansion board we are going to have a working 1565 external drive port -- well almost. I have made the 1565 port on the revF board use a 9-pin instead of an 8-pin mini-DIN, so that it can also power the drive. 

We'll call the enhanced port and external drive specification the "M1565".  The M1565 port is designed to talk to an original 1565 using a special adapter cable -- much like we changed the MEGA65 cartridge port from the C65's funny one (which is quite similar to the Plus/4 cartridge port) to a C64-compatible one to make life on the MEGA65 nicer.

But to test the M1565 port we need an M1565 drive! So I have designed up a little board that can connect to a conventional 3.5" floppy drive (or for that matter, a 5.25" one) and talk an enhanced version of the 1565 serial protocol.

The original 1565 protocol doesn't allow you to detect if there is actually a unit connected, nor to identify if its a 3.5" or 5.25" drive (the 1565 was designed only to ever be a 3.5" drive).  This would let us have a 1541-format compatible external drive that was as fast as the internal drive. It would also let me experiment with seeing how much data I can fit on a 5.25" disk some time, too :)

It would also be nice to know if the drive is a DD, HD (eg 1.44MB 3.5" or 1.2MB 5.25"), QD (eg SFD1001 style mechanism, if I remember correctly), or ED (eg 2.88MB 3.5").

We'd also like to support having more than one drive daisy-chained on the port, since the controller supports up to 7 external drives, and also to be able to write-protect the drive competely, e.g., if you are using it to archive disks.

This means we need some dip switches on the drive, as well some way to read those bits of information.

Fortunately the 1565 serial protocol has an unused status bit. I'm hijacking that to allow reading a serial bit stream, one bit per frame of data. This will also let us tell a 1565 from a M1565, as the 1565 will return all 1s, while this can only happen if the M1565 claims to be device 7 with a write-protected 2.88MB ED 5.25" floppy drive -- a drive that doesn't exist.

The logic for this is fairly simple, and I use the RESET pin on the 1565 port to synchronise this bit stream.  I'll go into more detail when I implement the communications protocol. For now, I'm just trying to get the PCB ready to send of for fabrication with a couple of other ones I am ordering, so the description here will be uncharacteristically short.

The other nice thing I have implemented is to make it work with either a twisted or straight-through floppy cable, since each board can connect to only a single drive.

This is what the prototype board looks like right now. Once again its designed to be all hand-solderable with no surface-mount components. 

Note that it is too long to fit in a real 1565 case, because that's not my objective right now.  The form-factor doesn't matter so much for testing. So I've just made the board the same width as a 3.5" floppy drive, and with those two bottom holes in theory lining up with the screw holes in the bottom of a 3.5" drive and placed appropriately that it should be possible to easily connect a short 34 pin data cable with the drive sitting there. We'll see how it goes when the boards arrive.

So that's really about it for now.  Hopefully I'll get the blog post out for the recent work on the expansion board and friends shortly as well.  There is still more testing required for that, but it's all looking quite promising, and some folks in the USA and Germany will be building their own prototypes of that soon to test, and will probably also want to build up one of these boards, too at some point :)

Oh, I should say that the KiCad design for this board can be found at  https://github.com/MEGA65/mega65-r3-expansion-board/tree/master/external_drive_board for the curious.