Data
Communications
Terminal
Line
Computer is
notified of overrun
Text-In Mode
Computer
Text Block
----I;.~---1---....,.-----_r_-----.,........-----~----____,
EeT +
(Terminal detects
an
incoming text
block larger than
its total buffer
.:ize.)
I
Computer should
attempt to retrans-
mit a number of
times then abort if
unsuccessful
In
response,
computer either
sends EOT (to put
terminal
in
CONTROL Mode)
or sends ENQ
awaiting
an
ACKO
or
ACK1
In
response the
CPU will send an
EOT (puts terminal
in CONTROL mode)
or send more text
Computer should
send next Text
block
or
EOT
-
-
WACK
(Terminal ack-
nowledges last Text
block but has no buf-
fer space left for more
Text blocks.)
I
RVI
(Terminal ack-
nowledges the last
text block but user
has requested CPU to
stop
sending.
I
'r
ACKOorACK1
(Terminal ack-
nowledges last
received Text
block as good)
,
ENQ
(Computer re-
sponds to WACK)
Computer
responds in one of
the ways listed
above (dependent
on content of
computer's last
rep~)
EeT
-
-
t
Last reply (Terminal retransmits its previous response)
I
t
Computer has no more data to send to terminal. Terminal
goes into CONTROL mode
TTD
Computer is tem-
porarily out of data
t
NAK
Terminal acknowledges delay and is ready for more text
J
Figure
7-20.
Operation of Block Protocol Control Characters(Continued)
7-43
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