Z80 home brew with FDC

[Eric Smith]

Alexis wrote:
> The FDC-1 uses a FD179x controller and it appears it uses the same 
> data clock and raw data inputs as the 765.

Not quite.  I don't recall the details of the difference, but I think it 
may have just been the polarity of one of the signals.  The 9229 and 
9239 had a configuration input to select between 179x and 765 modes. 
The 9216 would directly connect to one, and needed a small amount of 
logic (maybe just an inverter) for the other.

> The decoder is made up of a 74LS197 (clock divider), a dual 74LS74 flip-flop, 
> both used, a 74LS163 counter and an inverter. There are also some open 
> collector NAND gates to select the clock rate for either 8" or 5.25" drives. 
> It'll use more individual IC packages, but they're *much* easier to find.

Sure, but it doesn't sound like it's a very good data separator.  The 
good ones have a PLL (either analog or digital), because it is necessary 
to track speed variations, not just of the drive that you're using to 
read a disk, but also of the drive that wrote it.  Non-PLL data 
separators work OK when the disk is both written and read under optimal 
conditions, but are unreliable otherwise.

The 9216, 9229 or 9239 are *highly* recommended, as they have a good 
digital data separator.  The 9229 and 9239 also contain write precomp 
logic.  The 9239 uses higher resolution timing for its PLL, so it may 
perform better.