Z80 home brew with FDC

[Tony Duell]

> 
> Tony Duell wrote:
> >> I don't remeber who made the chip, but I made several diy z80 machines
> >> using a 279x.  The 279x had an internal pll data seperator.  All that was
> >> needed externaly was a cap and resistor as I recall.
> > 
> > I remember 3 external adjustments (1 trimmer capacitor, 2 pots) and a few 
> > fixed R/s and C/s, but nothing much more. Setting up is easy if you have 
> > a 'scope, you put the chip into a test mode (by grounding one of the 
> > pins) and then you get 3 signals on 3 of the pins each one of which is 
> > affected by one of the adjustments. So that, for example, you adjust the 
> > rimmer capacitor to get the right VCO free-running frequency, the VCO 
> > output is available on one of the pins in test mode.
> 
> Yep that's pretty much the same procedure that I have used with this chip.

Not suprising, I got it from the WD databook. HP don't regard these 
adjustments as being able ot be preformed in the field, but considering 
they provide a test jumper on most of the PCBs and labelled testpoints, I 
don't see what the problem is. (This is the same HP, I guess, that state 
that SMD parts are not field-replaceable...) But anyway..


> 
> > This chip was commonly used in HP HPIB and HPIL disk drive units, which 
> > is where I've had to set it up.
> 
> The Dragon 32/64's DOS cartrige also used the 2797, as did the two

Rigth. I know the CoCo rather better than the Dragon, IIRC the first CoCo 
disk interfaces used the 1793 and an external data separator (which is 
probably similar ot the circitry used in the TRS-80 M3 and M4 for obvious 
reasons), later ones used the 1773.
 
> prototype machines Dragon Data made just before they went to the wall in 
> 1984 :(
> 
> The chips can also deal with the 500K/s data rate that 8" and 1.44M 
> drives use.

The HP units I mentioned use the 600 rpm Sony 3.5" drives, so the data 
rate for a normal double-desnisty disk ('720K' to PC types) is 500kbps.

-tony