AMD
TAXI Technical Information Publication #89-03
S u b je c t : P ro p e r Us e fo r TAXI S yn c
Qu e s t io n :
What is the proper use for Sync? How often is a Sync needed?
An s w e r:
When a Transmitter has no data to send, it sends Sync. This symbol allows the
Receiver PLL to maintain phase and frequency lock with the transmitter, exactly as
would a normal DATA stream. It has the additional special property of being a unique bit
pattern that can be used to discover the byte boundaries in an otherwise continuous
bit stream.
The Receiver PLL takes some time to achieve phase and frequency lock (as described
in section 3.3). After lock is achieved the TAXI Receiver must align the incoming data to
the proper byte boundaries. The Receiver logic compares the incoming bit stream (bit by
bit and without regard for byte boundaries) with the pattern for Sync, and when it is
found, forces an internal bit counter to 0. The internal counter then continues to count
bits and run the byte rate logic without further reference until another Sync is found.
From this description of the Sync function, it is obvious that only ONE Sync symbol is
ever really required to define the byte boundary for the TAXI Receiver if the internal
counter continues to count correctly. It is unlikely that the internal logic function will
make a mistake, and therefore the counter will continue to count off the proper number
of bits per byte forever. However, there is some chance that noise can corrupt DATA
into a pattern that looks exactly like the Sync symbol. (The chance is about 0.13% of all
possible error types.) When this happens, the byte boundary is forced to an incorrect
position, and all data following is decoded incorrectly. TAXI Receiver violation detection
logic may or may not flag the errors, but the Receiver cannot distinguish properly framed
data from incorrectly framed data. The only thing that can correct this running error is
another Sync.
The minimum number of Syncs required in a user data stream is dictated by the system
sensitivity to running errors, and the system’s built in error detection mechanism.
However, it is a good practice to send a Sync every 1000 bytes.
Systems that send packetized data, should allow a Sync between each packet. This will
assure that if an error occurs, it will be terminated at the end of the packet and will not
corrupt the succeeding packets. Systems that send data at a rate slower than the TAXI
byte rate will have Sync automatically inserted as pad characters, so the user may not
need to specifically insert them.
Systems that send byte or short phrase data (commands or control words, for example)
might send a Sync before each byte or phrase to assure that the message is not missed
because of an earlier framing error.
From this discussion it should be clear that there is no RIGHT NUMBER of Syncs to
send with TAXI data. The correct number is dependent on the type of data the user is
sending, and the system sensitivity to running errors.
TAXIchip Integrated Circuits Technical Manual
97
AMD [ AMD ]
