User`s manual
0)
CD
@
@
TERMINAL
Sends a STRT (START) message which
means:
"I want to begin sending data
to you and the
!i:equence
numher
of
my
first
message
will
be
1."
______
-----""(2)
\....J
____
0
Receives STACK.
Sends Data
Messages
with a response field
set to 4 and the sequence field set to
I.
which means: "I am looking for your
message
I."
Other messages may
be
sent
at this time (i.e., messages
2,3,
etc.)
without waiting
for
a response.
_____________
0
(-;')
'\..J
Computer A receives NAK, retransmits /
Message
1 and any other messages sent
since (i.e.,
2,3,
etc.)
if
already
sent.--------..
o
/
Receives
ACK
and releases
Message
I.
Continues sending messages.
__________
@
Times out because
of
lack
of
response
------
for
Message
2.
Sends a reply for
_______
Me~ge2.
~G
Retransmits
Message
2 and
following messages.
DVII/PDP-Il
Receives STRT message.
Sends a
STACK
(Start
Ac!c'1ow!edge)
message which means:
"OK
with me;
here
is
the first sequence number (5) I
will
use
in
sending data messages to
you."
Receives Data
Message
I and checks
it
for
sequence and
CRC
errors. If there
is
a
sequence error.
go
to 12. If there
is
no
error.
go
to
Q
'"
f'Rf'
{'rror
wa<:
d!:'tected
f'nmpute
r
R
sends a
NAK
message with the response
field set to
0, which means: "All messages
up
to
0 (Modulo 256) have been accepted
and message I
is
in
error."
Sends
ACK
response
of
1 either
in
a separate
ACK
message or
in
the response field
of
a
data message.
Discard message and wait for proper
Message
2.
Send
NACK
response
of
1
in
the
response field.
Figure
B-5
DDCMP
Sample Handshaking Procedure
B-7