Looking for an 8 bit FDC...

[Tony Duell]

> 
> 
> OK, I'm wanting to build a board with an 8 bit CPU (probably Z80, 
> possibly 6502) and a floppy controller IC on board with the intention of 
> hanging it off my PC (via serial or parallel, undecided yet) and 
> allowing me to read and write *most* formats from various 1980's 8 bit 
> micros...
> 
> Intel's 8271 looks like a possibility at the moment, but I thought I'd 

I would avoid the 8271, if only becuase it's somewhat rare (the BBC micro 
being the only common machine to use it). And there's the thing that I've 
never seen an Intel LSI chip without a design misfeature ;-)

I'd go for one of the Western Digital ones. If you're lucky you'll find a 
2793 or 2797 or something like that. Everything, including the data 
seperator, is on-chip. Setting up is easy. 

The 1770 (or 1772) or the 1773 aee other easy-to-use chips. 

Be warned that the 1771 is a bit odd. It's FM only _and_ it can write a 
Data Marker than no other normal floppy controllers can. The problem is 
that that data marker was used on the TRS-80 Model 1 (on the directory 
cylinder only). If you want to copy disks from that machine, you need the 
1771 (to the extent that the double-density upgrades for the Model 1 
added a 1791 or similar, and kept the 1771 alongside it).

> poll the list for alternative ideas too. FM support is of course 
> critical - MFM is less of an issue as the host PC can handle that.
> 
> I've never built any kind of computer from scratch, so it'll be a useful 
> experience. I figure on putting just enough code in ROM to support 
> downloading of actual firmware to the device over whatever the link is 
> to the PC, as that should save a lot of headache!
> 
> Hopefully RAM requirements will be low enough that I can go the SRAM 
> route and avoid messing around with DRAM refresh (although IIRC the Z80 
> has much of the necessary stuff built in...)

32K*8 bit SRAM chips are common. 128K*8 are also common (but maybe harder 
to find in a DIP package). You really don't want to be doing 
bank-switching of memory, do you? In which case, 2 32K chips would fill 
the address space of a Z80 or 6502. 

Personally, I think 32K RAM would be enough, along with, say, 8K (or 
16K) EPROM. Many 8 bit machines supported a floppy drive with far less.

-tony