On 1/24/14 9:36 AM, John Wilson wrote:
Almost definitely showing my own ignorance (I'm
very new to serial comms),
but my impression is that in the old days, synchronous ports *always* used
a modem-supplied (etc.) external clock signal, and it's only newer fancy-pants
ports like the Zilog Z85(2)30 that try to be cute about using a PLL to derive
a clock from transitions in the bit stream.
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. Think of the distinction today between the PHY and MAC in an Ethernet interface.
There is also the notion of a 'modem eliminator' which is a box that will
generate
a clock for you in environments where you don't need physical modems.