On Jan 24, 2014, at 1:56 AM, Chuck Guzis <cclist at sydex.com> wrote:
...
Synchronous transmission has neither start nor stop bits--after synchronization with the
transmitter is achieved, any gaps in the transmission are filled with "idle"
characters, whose sole purpose is to mark time (just like the "stop
bit")--they're discarded by the receiver.
Almost always that is true. There is at least one oddball exception: the 21-bit
synchronous output stream for the PLATO terminals. That consists of a start bit, 19 data
bits, and a parity bit. Idling is done by sending NOP commands, which have all zero in
the data bits (but the standard start and parity bits).
I suspect this was done to provide one?s density for the receiver to lock onto. The
standard way to achieve that in sync communication is with non-zero idle characters (DDCMP
and Bisync) or bit stuffing (SDLC, HDLC). Or if you view things like Ethernet as sync
communication, which would make sense, there is also encoding that always has transitions
(Manchester in 10 Mb/s Ethernet), coding with guaranteed one?s density (4b/5b in FDDI and
100 Mb/s Ethernet), or coding with that plus DC balance (8b/10b in Fibre Channel and 1Gb/s
Ethernet).
paul