🚀 go-pugleaf

Dear Keith,
 
Just some notes from the trenches in dealing with the Mac serial ports.
 
If something holds the interrupts off for more than 3 character periods (about
3ms @ 9600 bps) the SCC will overrun.  The Mac OS has hooks in some interrupt
routines to try to unload the SCC manually, and to insert the received chars
into a buffer that will be unloaded by the driver the next time the driver is
called.  If you are not doing anything special to the SCC setup, or are not
writing your own driver this _generally_ works.  Apple however loudly declaims
(and disclaims) that the Mac is not a real-time device and that you can't count
on how long or when interrupts may be off.  Disk activity and network activity
(LocalTalk particularly, for obvious reasons) can shut the interrupts off for
long enough that you can drop characters.
 
If you are doing something silly with the SCC all bets are off.  We have a
special driver that when the SCC is properly configured for the data stream
that we are looking at (which is broadcast, we can't handshake it), the receipt
of a character doesn't set the bit on the VIA that the little hooks are looking
for.
 
If you are not messing too badly with the SCC, but are using your own driver
you may be in better shape.  DTS sent me a copy of the serial driver in the 64k
rom machines as an example of how to to use the PollProc hook (what they call
that little stack of characters).
 
You probably just should do handshaking if you can.
 
Good luck,
 
Bob Myers