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TAXIchip Integrated Circuits Technical Manual

TAXI Technical Information Publication #89-03

Subject: Proper Use for TAXI Sync

Question:

What is the proper use for Sync? How often is a Sync needed?

Answer:

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.

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