Wah wah wah waaaaaaah. In the battle for 5.25 inch Apple II disk write emulation, it seems I declared victory too soon. While write support works great in many situations, it doesn’t work at all in others. The problem is with the /WRREQ signal from the Apple II, which goes low to indicate when a write is occurring. On old-school disk controller cards, like the Disk 5.25 Controller Card, or the even older Disk II Controller Card, this signal is generated by a 74LS05 hex inverter chip with open collector outputs. It can actively pull the signal low, but it needs an external pull-up resistor to pull the signal high. And Floppy Emu doesn’t have one. On the Apple IIgs (and probably also the IIc), the circuitry can actively drive /WRREQ high, so this isn’t a problem. And on other Apple II models, it’s also not a problem if there’s another real drive connected in the daisy chain, because that drive will have a pull-up, and /WRREQ is shared among all drives. But if Floppy Emu is the only disk drive connected to a Disk 5.25 or Disk II Controller Card, writes won’t work. Ouch.

The logic analyzer trace above shows what happens. /WRREQ is connected directly to an input pin on the Xilinx XC9572XL CPLD. The input has a very weak pull-up that’s active at initialization time, as well as a keeper circuit that actively maintains the input at the last externally-driven level. At power-up, the weak pull-up pulls /WRREQ high, and the keeper circuit keeps it there. Then when a write occurs, the disk controller card actively drives /WRREQ low. At the end of the write operation it releases /WRREQ, but with nothing to pull it high again, the keeper circuit maintains the low value forever. To the Floppy Emu, it looks like a write operation that never ends. You can see the result in the trace. /WRREQ stays low forever, even after the drive enable signal is de-asserted.

Unfortunately, I don’t think there’s any way to fix this other than to add a pull-up resistor. The XC9572XL CPLD chip doesn’t have configurable built-in pull-ups that I can turn on in firmware, as far as I can tell. 5.25 inch read emulation will still work fine, as well as 5.25 write emulation on a IIgs or on any Apple II with another drive present. But for a single-drive Floppy Emu setup on an Apple IIe, write emulation won’t work without a hardware fix.

I already designed an adapter board, which I described in this earlier post. I had hoped that the adapter board would only be needed for certain situations of 3.5 inch Apple II disk emulation, but now it seems certain situations of 5.25 inch emulation will need it too. There’s too much “certain situations” about the whole thing for my comfort, and I’m sure it confuses everyone else just as much. So I’ll probably just rechristen the adapter board as a general purpose “Apple II adapter”, even though it’s not strictly needed for all circumstances of Apple II emulation. That will be easy to understand, and won’t steer anybody wrong. Power users can read the footnotes and discover which Apple II setups can work without the adapter board, if they wish.

Adding a pull-up resistor to the adapter board will be easy, but it does mean I’ll need to have new PCBs made. The only good thing in all of this is that it justifies my decision to postpone manufacturing large numbers of the adapter boards until after the Apple II 5.25 inch and Smartport disk emulation is working. If I had built lots of adapter boards three weeks ago when I thought they were only needed for 3.5 inch Apple II disks, I would now have a lot of angry people who’d bought them, and a pile of additional boards I couldn’t use.