How to add 3rd and 4th floppy drive to WD37C65 or FDC9266

[Laszlo Szolnoki]

Hello everyone,
I want to use 3 floppy drives in my CP/m system.  The current controller boards use PC-AT connectors with two drive select and motor on signals each. The old Shugart connector would allow the use of 4 drives. I have not been able to find a schematic on the internet that could be reused. If anyone has a clue or even better a schematic, I would appreciate feedback.  

Cheers

Laszlo

[Sergey Kiselev]

Hi,

Both WD37C65 and FDC9266 can do 4 floppy drives.

My designs that use WD37C65 [Zeta SBC V2](https://github.com/skiselev/zeta_sbc) and [Flock](https://github.com/skiselev/flock) operate it in AT/EISA mode. In this mode, the FDC only supports 2 drives, as it replaces third/fourth drive selects with motor enable signals.

You could potentially choose to operate WD37C65 in "special mode" (see page 25-21 of the [WD37C65C datasheet](https://github.com/skiselev/flock/blob/main/Documentation/WD37C65C.pdf)).

It might require some changes in the schematic and will require changes to the software.

FDC9266/FDC9268 might be a bit more straight-forward as far as connecting 4 drives goes, but it requires more external components. My [ECB DiskIO V3](https://www.retrobrewcomputers.org/doku.php?id=boards:ecb:diskio-v3:start) design supports for 4 drives, and can be easily adopted to any Z80 system (throw away bus transceivers / buffers if you don't need them... most RC2014 don't use them, and somehow it still works).

If you plan to use PC-style floppy drives that are preset to the second drive select (DS1 or DS2 depending if you're counting from DS0 or DS1), and PC floppy cables with a "twist" between the second and the first floppy, I can recommend using two 34-pin connectors on the controller, and connect DS0, and DS1 signals to the first connector in PC pinout, and connect connect DS2 and DS3 signals to the second connector. My schematic provides a single "motor" signal. So that all floppy drives motors will turn on when one of them is accessed. If you don't like this behavior, you can add another latch, and have separate motor signal for each floppy.

Yet another option is to use two WD37C65's in AT/EISA mode... You'd need to check that the BIOS supports that. I am pretty much sure RomWBW does not support two FDCs at the same time.

Finally, you can also use Intel 82087A or NEC PC8477 FDCs.  I'd think that the design will be similar to WD37C65 design, e.g. [Flock](https://github.com/skiselev/flock), but you wouldn't need to provide separate chip selects for CCR and DOR. You'll still might need a contraption with DACK or TC signals connected to a one bit out port or a chip select to indicate "transfer complete" at the end of data transfers. You can refer to my [Monster FDC](https://github.com/skiselev/monster-fdc) project as an example for connecting 4 floppy drives to one controller.

Thanks,

Sergey

[Laszlo Szolnoki]

Thanks for the feedback, Sergey. I needed a reconfirmation on my initial thoughts. I will give the "special mode" version a go because now I have a working WD system. This will make troubleshooting easier. Then I'll rework my FDC266 board. My goal is to run a system with one 3.5" and two 5.25" disks. As soon as I make progress, I will report back again. Cheers Laszlo 

[Alan Cox]

Another approach might be to just wire the drive select lines to an
external latch and then ignore the ones on the 765 itself. I think
that will work looking at the docs briefly. It would need some trivial
changes to the driver code but shouldn't be hard to do 8 or more
drives that way.

Alan

[Laszlo Szolnoki]

I am going to build a WD37C65 board with 2 floppy ports. One for PC/AT and one with 4 drive selects. I will use a buffer for MotorON. All other signals can be shared, as far as I see.

Laszlo