CP-M Z80 home brew computer circuit board

Tony Duell ard at p850ug1.demon.co.uk
Wed Jun 18 22:32:46 CDT 2008

> The 279x (1,3,7) series chips actualy had a working pll data seperator.

I thought there were more varients than that, but I don't haev the WD 
data sheet in front of me. The PLL is quite easy to set up (there's a 
test pin on the chip that you have to ground after reset IIRC, and then 
you get 3 wavefroms on 3 pins of the chip that have to set to the right 
frequency or pulse width using the 3 tweakers), but you need a 'scope.

FWIW this chip was used in some HP floppy drive units, permanently 
strapped for MFM (double-density) use. 

> Here is a link to the 279x controller chip data sheet:
> http://retrotechnology.com/herbs_stuff/fdc_datasheet.pdf
> As I recall, the 179x series chips could not reliably do sector reads 
> from disks formated on a 177x controller.  The 179x could do a track 
> read of the disk.  Morrow provided a program with the Disk Jocky 
> controller which would do track reads, then write
>  the track back out creating a disk which worked well with either the 
> 177x or 179x series controllers.  It was not a common issue, as very few 
> people used single density once double density became available, and new 
> single density disks worked well with either controller.

Eh? The 1770, 1772 (== 1770 with different step rates) and 1773 can all 
do MFM operation, The 1771 is FM (single density) only, and has the 
feature of beine able to write some odd data markers.

I have never had any prolems reading disks between a CoCo (using a 1773 
controller) and a Model 4 (1793 controller). The only problem comes with 
the Model 1 (1771), which did use one of those strange markers on the 
directory track.


More information about the cctalk mailing list