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MULTIPOINT PROTOCOLS WITH
13260C OR 132600 COMMUNICATIONS
ACCESSORY
Block protocols transfer data in blocks. The blocks are
made up of three parts:
•
Block framing characters.
•
Text
(l
to n characters, where n depends on terminal
configuration).
•
Block check character(s).
This data format is always present in block protocols. In
addition, block protocols use special character sequences
to control all data transfers.
A block check character is included at the end of each data
block. If a data error is detected the protocol will normally
automatically attempt a retransmission of the block.
Note: Since the Multipoint buffers use display memory, a
minimum of 8K of display memory is recom-
mended.
CHARACTER MODE TRANSFERS
Character mode transfers are not permitted with block
protocols. All data transfers are implemented using a
block data structure.
MULTI CHARACTER TRANSFERS
When the terminal makes a multi character response of
fixed length to the computer (status etc.), the data sent
will be in the form of a block with framing characters and
one or two block check characters.
MESSAGE BLOCKS
A message consists of one or more blocks of text data. The
use of blocks enables the terminal to efficiently buffer
data, respond to transmission errors and guarantee data
integrity. Maximum block size is strap selectable and
permits you to use the size best suited to computer re-
quirements. (Refer to Buffer Size.)
BLOCK OPERATION
The block protocol is designed to operate using either
synchronous or asynchronous communications. Data
transmission is done in multiple character blocks. The
block size used is limited by the terminal's communica-
tions buffer (refer to Configuration).
The input buffer size limits the size of the data block that
can be sent to the terminal. For example, if the input
Data Communications
buffer size is 500 bytes, sending a block of data larger than
500 bytes will result in a loss of data. If this happens an
EOT character will be sent to the computer.
Two forms of text blocks are shown in figure 5-11. The first
is a block received from a computer. Note that no ID
characters are used since the terminal or· terminals to
receive the data have already been identified by a select
sequence. The second block is one sent from a terminal. In
multipoint configurations, since more than one terminal
may have been polled, the first text block sent from each
terminal must have the terminal ID included. The ID
characters are not repeated (as in poll and select sequ-
ences) since they are included in the block check charac-
ter.
Included in BCC
/
\
<\>
<TEXT><~/\>
<BCC><PAD>
\
/
Included in block size
(a) Received from the Computer
Included in BCC
/
\
<\>
<GID.><DID.><TEXT><~/\>
<BCC><PAD>
\
/
Included in block size
(b) Sent by the Terminal
• Multipoint only
Figure 5-11. Examples of Block Transmissions
TEXT TERMINATION. When the terminal is receiv-
ing text (Text-In mode) it will accept only ETB (octal 27),
ETX (octal 3), or ENQ (octal 5) as a text block terminator.
An ETB indicates the end of a block with one or more
blocks to follow. An ETX indicates the end of the current
block and the end of the text transfer. An ENQ character
indicates that the current block has been aborted. The
terminal will respond to the ENQ with an NAK to request
the retransmission of the aborted text block. When the
terminal is sending data (Text-Out mode), it will termi-
nate text blocks with either an ETB, or ETX character.
The terminal may send a STX ENQ as a Temporary Text
Delay (TTD) notification. This indicates that there is more
text to come but that it is not ready to be transmitted (i.e.,
it is still being read from a cartridge tape). A TTD should
5-25
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