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Data Communications
Table 5-11. Summary of Block Protocol Control Characters
CONTROL
TEXT-IN
TEXT-OUT
STX-'TEXT"-ETB-ETX
"EOT"
"ENQ"
"RVI"
"ACKO/ACK1 "
"WACK"
"NAK"
POLL
RESPONSE
Positive response
to POLL.
Negative response
to POLL. Termi-
nal has no TEXT
to xmit.
SELECT
RESPONSE
Terminal
tells
CPU that it is
ready to accept
TEXT (ACKO).
Term is temporar-
ily busy (term has
no available buf-
fers). Cannot ac-
cept TEXT.
RECEIVED
Sent by CPU as a
response to an
ACK
received
from terminal.
CPU has no more
TEXT to xmit to
terminal.
CPU requests ter-
minal send last
TEXT
acknow-
ledgement.
TRANSMITTED
Terminal has de-
tected data over-
flow. (This may
only be a tempo-
rary condition, if
the size of the
transmission does
not exceed the
size of the termi-
nal input buffer.)
Terminal acknow-
ledges last text
block and requests
the CPU to stop
sending ( . . ).
Terminal
tells
CPU
that
last
TEXT block was
received OK.
Term
acknow-
ledges last TEXT
block received. OK
but now term has
no more buffers
&
cannot
accept
more TEXT.
Term detected er-
ror in last TEXT
block CPU sent.
Invalid VRC/BCC
or frame chars.
RECEIVED
CPU has decided
to abort terminal
xmission.
CPU
acknow-
ledges last TEXT
block
&
requests
term send "EOT".
CPU tells term
that last TEXT that
term sent was OK.
OK.
CPU
acknow-
ledges last TEXT
block sent by term
but tell term to wait
because
CPU
does not have any-
more buffs.
CPU detected er-
ror in last TEXT
block term sent.
Invalid VRC/BCC
or frame chars.
TRANSMITTED
Sent by terminal
as a response to
an ACK received
from CPU.
Term has no more
TEXT to send to
CPU or has just
received an "RVI".
Term
requests
CPU
retransmit
last
acknow-
ledgement
to
TEXT.
STX-GID-DID-CN-ETX
. . has been pres-
sed. Any data that
is waiting to be
sent to the CPU
is lost.
STX-ENQ ("TID")
CPU is temporarily
out of text. The
terminal must re-
spond with a NAK.
Term is temporar-
ily out of data.
CONTROL MODE
COMPUTER
LINE
TERMINAL
PO L L Sequence
•
~
~
+
No response. (Terminal
EOT (Terminal has no data
Text Block !Terminal
STX-GID-DID-CN-ETX
does not recognIZe PO L L
to send In response to POLL.)
responds to PO L L by
Terminal_key has
sequence as valid for
I
transmitting teXT. terminal
this terminal.)
goes Into TEXT·OUT
been pressed
I
mode.!
I
I
SE LECT Sequence
+
+
+
No response. (Terminal
WACK
(Terminal
IS
ACKO
(Terminal
does not recognize SE LEeT
temporarily not
acknowledges that
sequence as val id for
able to accept
it
is ready to
this terminal.)
Text because of
accept Text; terminal
I
lack of buffer
goes Into TEXT·IN
space.)
mode.1
I
I
Figure 5-12. Operation of Block Protocol Control Characters (Sheet 1 of 2)
5-32
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