AM3270/TR3271 Reference Manual

What are Bisync Protocols?
Bisync Protocols—Concepts and Terminology
086705 Tandem Computers Incorporated 1–3
3. Data Block Transmission:
The message block is then sent. The beginning of the message or control sequence
is located by finding an appropriate control character following the SYNs. These
would be control characters that are defined as beginning a block of data or a
control sequence, such as SOH or STX.
4. Synchronization Maintenance/Checking:
When the data block is very large, the transmitting station may send additional
sync-idles (such as a SYN SYN for normal data, or a DLE SYN for transparent
data) at one-second intervals to maintain character synchronization between the
devices during transmission. The transmit timeout rate defines the rate that these
additional characters are inserted in the data.
The receive timeout limits the time a transmitting station will wait for a reply,
limits the time the receiving station will check the line for sync-idle characters, and
sets the maximum time that a tributary station on a multipoint circuit may remain
in control mode.
5. End-of-Block Indication:
The last block in a message ends with an ETX character followed by a BCC. All
other blocks in the message end with either an ETB or an ITB, followed by one or
more BCC characters.
Pad characters might also be sent to insure complete transmission or reception of
the first or last significant character of a transmission. If they are sent to indicate
the end of the block, they will cause failure retries and recovery attempts.
The Message Block The basic unit of communication for bisync protocols is the message block, which is
also called a “frame.” The basic message block is shown in Figure 1-2.
In this manual the term “multiblock” refers to long messages that have been separated
into sections for error detection purposes. The separated sections use the Intermediate
Transmission Block (ITB) control character to tell the receiving device that it does not
need to reply until it receives an End of Transmission (ETB) character.
Each block contains text and a trailer, and may begin with a header. To identify each
of these message block elements, various control characters are used, such as Start of
Text (STX) and End of Text (ETX). Two or more synchronization characters (SYN)
must precede the message block, to establish timing coordination between the sender
and the receiver.