Patrick Schaefer wrote: > Bryan Villados schrieb: > > > I'm curious about the incompatibility between an Apple II 5.25" drive > > and a Macintosh Plus... Is it incompatibility on an electronics level > > (wires don't match up, voltage, etc.), or is it more because of a lack > > of a software driver for Mac OS? > > This is mainly a software problem, the electrical connections are the > same. Not 100%, but they are compatible enough for most purposes. The Mac has some extra features, such as being able to pump a clock signal out one of the pins, which is used to control the rotation speed of the old 400K and 800K drives. The Apple 3.5 Drive has built-in speed control. Certainly all the important signals (power, ground, phase outputs, read/write data, write enable) are the same. > However, the Mac Plus uses an IWM chip instead of discrete logic, > and this controller chip supports much more features than the old > Duodisk card. I don't know enough about the low-level details of the IWM to be confident about this issue, as the only detailed documentation is the Apple IIgs Hardware Reference. Mac-specific documentation on the IWM is sparse at best. The main obvious difference between how the IWM works in the Mac and Apple II is that it uses a nominal 8 MHz clock in the Mac, while it uses the nominal 7 MHz clock in the Apple II. There is a mode register bit which tells the IWM how many cycles to count to get its time base (7 on the Apple II, 8 on the Mac), so it ends up working at the same data rate on both macines (apart from a small percentage difference). There is one key point about using the 5.25" drive and intelligent SmartPort devices (like the UniDisk 3.5) which might prevent them from working on the Mac: the IWM must run in a "synchronous" mode, and the CPU has to use exactly the right number of clock cycles between write operations (32 cycles on the Apple II). This might not be possible on the Mac, since the 680x0 processor is not tied as closely to the hardware as the 6502/65C02/65816 in the Apple II. By comparison, the Apple 3.5 Drive (and Mac drives) use the IWM in an "asynchronous" mode, which doesn't require as precise timing, only that the CPU can keep up (for both reading and writing): a byte is read or written every 16 microseconds on 400K and 800K disks (also for 720K MFM disks with the SWIM chip and a SuperDrive; 1.44M MFM disks transfer a byte every 8 microseconds). I imagine that the Apple HD20 works on the same principle as the 400K and 800K disks (with the IWM in async mode), but I haven't seen any information to confirm this. -- David Empson dempson@actrix.gen.nz Snail mail: P O Box 27-103, Wellington, New Zealand