Autzoo.1296
hacknews
utzoo!henry
Wed Jan 20 01:30:10 1982
DZ hangs
For those who found the previous message a bit cryptic...  The problem
is that the BREAK bits are getting set on one or all of the lines on
the affected DZ(s).  This is tricky to spot, because those bits are in
an (ugh!) write-only register.  Said bits force the lines into a BREAK
state, overriding normal output without any error indication.  Nothing
in the current software ever resets those bits.

I am not yet sure what exactly causes the problem, although the process
that scans the DZs for carrier-status changes is suspect:  the last hang
seems to have happened just as the CSRG line got hung up (which is what
started me thinking along these lines...).  The trick is that the carrier
register on the DZ is the read-only register that occupies the same bus
address as the break register;  all it takes is a read-modify-write
instruction on that register and the break bits get set.  Why it affects
only particular DZs, and why carrier-change events on the fifth DZ screw
up the fourth DZ, isn't yet clear.