CP-M Z80 home brew computer circuit board

Eric Smith eric at brouhaha.com
Thu Jun 19 14:33:29 CDT 2008

Les wrote:
> As I recall, the 179x series chips could not reliably do sector reads from
> disks formated on a 177x controller.

The 179x can read disks formatted on a 1771 just fine.  What the 179x can't
do is distinguish nonstandard data marks.  Most people using the 1771
were smart enough to use the standard data marks.  Unfortunately, TRS-DOS
for the Radio Shack TRS-80 Model I did use nonstandard marks.

The IBM-standard formats have two different data marks, for normal
data and deleted data.  The 1771 and 179x can both read and write
those data marks, and on read, distinguish between them.

However, the 1771 also supports two non-standard data mark patterns,
which it can write, and distinguish on read.

On the 179x, it is not possible to distinguish the non-standard data
marks from the standard ones.  It's possible to write the non-standard
ones during formatting, but not when writing a sector.

This is why the "doubler" products for the TRS-80 Model 1 didn't
completely replace the 1771 with a 1791, but instead put them in
parallel and added switching logic.


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