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Data Communications
TEXT-OUT Mode
LINE
NAK (Computer
detecT~
bad VRC
01
IllVdlld Illock check Ch,HiJctel
,J
RV I (Computer .. cknowledges Text block
as good. but tells terminal to stop.)
1-----------~-______1,... Terminal sends EOT and goes
Into
CONTROL Mode
COMPUTER
TERMINAL
--------.--"""'T'"-"T'""""""T-------+---folt-------------
Texi block
f2
CKO
0'
ACKl Icamt,p,
EtT
ICampu'e, d",de,
<iCknowlf'cqes
T~)(t
Illor.k
rJ'>
to aboft current lerm:nal
9
U O(l.)
operatlonl
Terminal s!Ops
operation
1----------1~--+-
and goes 11110 CONTROL Mode
WACK (Computer ilcknowledqrs
Id\t Text lliock but tell.,
lermilldl thdt computer hd')
'10
more buffer spJce for
additional Text.)
~ - - - - - - - - - - -
_ _+_-_+_
Terminal transmits next Text
block. or
EaT
If all Text
hlocks transmitted and
terminal goes Into CONTROL
MOOE
Terminal sends ENQ
ENQ
ITermlflal response
to WACKl
LdSr R~ply,
(Computer
retr<Jn.,mlls Its preVIous
reply,)
E~T
(Computer deCides
to
JIJort
CUffel)! terminal operation.!
IL-
+--_+_
;~~~llr~ta~ ~~~\o:~r~t~~~da;d
- - - - - - - - - - - - - - - - - . - - -.....t--+
Termlndl
responds
In Qnl' of
the way" listed ahove
(tiepenllent on content of
computer's last replyl
~
NAK
I
+
Computer Interprets EOT JS the
terminal ha\/lng
110
more Text to
~end
EOT
lA11 Text blocks completed
terminal goes Into CONTROL
Model
TTD
Terminal is
tem~rariJyol.C
at data
"the
terminal is
ready
;t
sends
more
text.
~
not.
TTD
is
sent.gain to
the CPU
TEXT·IN Mode
COMPUTER
LINE
TERMINAL
Tf'xt block
--------+--t-;'~-----_r------.,~----.,..---..,
EDT
lTermlrlJI detects
I 0
overflow dill! qoes
Illto
CONTROL Mode)
WACK
(TermlnJI acknowledges
I
.
last Text block, but has no
Computer
IS
not died o f . 1
Iluffer space left for more
overflow
- . . - 1 - - + _ '
Text blocks.)
Computer should attempt
to retrdnsmlt d number
of times, then abort If
unsuccessful
NAK
lTermllldl detects
bad VRC or Invalid hlock
check ehO"re'l
RVI (Terminal acknowledges
the last text block but
user has requested CPU to
stop sending - . . ).
In response, computer
eiTher sends EaT (to pu""',
_~_-+
- - J
termlndl In CONTROL Model
or sends END awaiting an
ACKO m ACKI
In response the CPU
will send an EDT (puls
terminal in Control mode)
or send more text.
Computer should send
next Text block or EDT
ENQ
(Computer re.,ponds to WACK)
Computer responds In one
of the ways listed allo .... e
(dependent on content of
computer's last reply)
•
EQT
+
Last reply. (Terminal
retransmits its previous
response
I
+
Computer has no more datd
to send to terminal. Terminal
goes Into CONTROL Mode
ACKO
or
ACK 1
(Terminal
acknowledges last
received Text block
as good)
I
TTD
Computer
II temporl1i1y
alA 01 data
t
NAK
Terminal .:::knowleOges;
deloy
and
is ru<ly
br
more text
I
Figure 5-12. Operation of Block Protocol Control Characters (Sheet 2 of 2)
5-33
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