24 Jan
2014
24 Jan
'14
4:50 p.m.
On Jan 24, 2014, at 6:48 PM, John Wilson wrote:
On Fri, Jan 24, 2014 at 10:14:39AM -0800, Al Kossow
wrote:
It's dual purpose. Believe me, you need guaranteed transitions to keep
your PLL synced; PLLs have MUCH more drift than crystals, and you
couldn't use a crystal to send NULs all day long either. The fact that
it allows you to send special out-of-band symbols like the flag and
abort sequences is a nice side effect of the zero-stuff regime, similar
to the break sequence in async comms (which is otherwise a framing
error).
- Dave
Manchester-encoded transmission schemes have
embedded clocking. These go back
well before the days of the SCC. The on-wire encoding was normally hidden by
whatever modem was handling what was on the wire, so all you would see would
be clock and data.
Well, exactly. So clock insertion/recovery is the modem's business, and has
already happened by the time you get out to the computer's serial port and
are dealing with things like sync and idle characters. So I still claim
that sync/idle chars have to do with byte framing, and SDLC bit-stuffing is
for packet framing, and neither one has anything directly to do with PLL sync.
Once you establish byte framing with a couple of SYN characters, you can
send NULs all day long w/o losing your place, and they'll all be dutifully
clocked into the receiving port...