AM3270/TR3271 Reference Manual

Control Characters
Bisync Protocols—Concepts and Terminology
086705 Tandem Computers Incorporated 1–5
Elements of a Message Block
These are the elements of a message block:
Header
Identified by a Start of Header (SOH) character at the beginning and a Start of Text
(STX) at the end. Contains one or more characters identifying the sending and/or
receiving location, which also may be referred to as an address.
Text
Identified by a preceding Start-of-Text (STX) control character. Single-block
messages are followed by an End of Text (ETX) control character; in multiblock
messages only the last text block is followed by an ETX.
The text portion of the block is variable in length. The maximum length for 3270
BSC is limited by buffer space The block contains transparent data when that data
is delimited by Data Link Escape (DLE) characters followed by an STX character.
DLE characters turn the transmission control character recognition logic of the
receiving device on and off.
While the block length is variable, the maximum size is limited by the specific
application of a bisync protocol. Two factors to consider when setting the
maximum block length are:
Availability of buffer space to store the block
Increasing error rate as block size increases
In general, the longer the message block, the more likely that an error will occur
during transmission.
Trailer
Consists of a block-checking character (BCC) that contains a checksum for error-
checking. The method of error-checking depends on the character code used. For
ASCII code, either a vertical or longitudinal redundancy check (VRC/LRC) is
used. For EBCDIC and six-bit transmission (SBT) codes, a cyclic redundancy
check (CRC) is used. AM3270 and TR3271 do not support SBT.
These calculations result in the block check character(s), which are then
transmitted as BCCs. Block-checking and error checking are discussed in more
detail later in this section.
Transmission Control
Characters
There are two types of bisync control characters, one for terminal control and one for
transmission control. Terminal control characters control various functions on the
terminal such as line feed. Transmission control characters, also called line control
characters, determine how the data is sent and received and how errors are detected.