Block Mode Transfers - HP 262SA Reference Manual

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Data Communications
Block Mode Transfers
All data transfers between the host computer program and any of the terminals in the
multipoint configuration employ data blocks made up of the following three parts:
• Block framing characters
• Text (1 to n characters, where n depends on the terminal configuration)
• Block check character(s)
The block check character (BCC) is used to verify that the data was received without error. If a
data error is detected, the protocol will normally automatically attempt a retransmission of
the block.
The block protocol is designed to operate using either synchronous or asynchronous communi-
cations. Data transmission is done in multiple character blocks. The block size used is limited
by the size of the terminal's communications buffer.
Two forms of text blocks are shown in figure 7-19. 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 sequences) since they are included in the block check character.
(a) Received from the Computer
Included in BCC
/ \
(\)
(TEXT)(~/~)
(BCC)(PAD)
\ /
Included in Block Size
(b) Sent by the Terminal
Included in BCC
I \
(\)
(GID)(DID)(TEXT)(~/~)
(BCC)(PAD)
\
/
Included in Block Size
Figure 7-19. Examples of Block Transmissions
7-31