24 Jan
2014
24 Jan
'14
3:55 p.m.
On 01/24/2014 01:33 PM, Paul Koning wrote:
On Jan 24, 2014, at 1:14 PM, Al Kossow <aek at
bitsavers.org> wrote:
Manchester was only one of many. People believed that NZRI could not
extract clock data at one time.
On 1/24/14 9:36 AM, John Wilson wrote:
Manchester encoding is an excellent example of a modulation scheme that provides
bit clock as part of the demodulation process. Given the required bandwidth, it?s a good
LAN (or tape, e.g., DECtape) scheme, not so much for long distance connections. 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.
Some other simple modulation schemes are not clocked;
FSK is a good example (110 baud, 300 baud, and Bell 202 modems). But
Generally
they didn't provide clocking but micros and cassette tape
forced people to recognize that it can self clock
if your willing to do the work.
many others, and in particular any of the faster phone
modem schemes (1200 bps and up) use modulation where the demodulator recovers the bit
clock as part of (or in order to do) the demodulation. PSK is an example, and certainly
anything harder like QAM inherently depends on having a good receiver clock.
Most
techniques required a known starting point and then worked out how
or around how to get back to that point.
Allison