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SERIA LIZE -DESERIALIZE
Characters are sent over the communications lines
serial-by-bit; while, within the 2740/2741, charac-
ters are transferred parallel.-by-hit. The serializer-
deserializer (serdes) changes the character from one
form to the other.
Character Synchronization
The 2740/2741 Communication Termina1s use start;
stop synchronization to identify bits
on
the line. The
line
has
t\\'o conditions: bit and no-bit. Before and
between characters, the line is in a bit condition.
A
character is started by a transition from the bit to
the no-bit condition. From this transition to the first
bit time is the start bit (Figure 23). Following the
start-bit are the seven bit-times for the character in
this sequence: B-A-8-4-2-1-C. After the C-bit
time, the line is returned to the bit condition. From
the end of the G-bit to the next start-bit is the stop
bit. The receiving unit uses the start transition to
start a clock
to sample the condition of the line in
the middle of each bit-time. The clock is stopped
when the character is completely entered in-the ser- .
des (serializer-deserializer) register, then restarted
by the next start transition.
Serializer-Deserializer Shift Register
The serializer-deserializer uses a nine-trigger shift
register (a trigger for each of the data bits ,check
bit, start bit, and stop bit) to convert a character
from serial-by-bit to parallel-by-bit (deserialize)
and from parallel-by-bit to s3rial-by-bit (serialize).
IBM
27-10
LINE CONTROL
In terminal operations with a multiplexer or other
terminals, a control discipline is required on the
communication line. This discipline is called line
control, and one of its functions is to prevent two or
more stations from attempting to use the line simul-
taneously (line contention).
Line Control Signals
The line control signals (EOA, EOT, EOB, Yes, No,
SOA, and Inquiry) are· represented in a shorthand
form
@,©,@,.
®,
®,
@,
and
®,
respec-
tively). This form is used in programming, as well
as in communications and line control discussions.
Also, the terms response and answer are used in the
following restricted manner.
, 22
Answer. The negative, positive, or no answer at
all to a check
(VRC
or
LRC)
following an
EOB.
Start Transitianl
.~
No-8lt
line
Serdes Clock
Puises
2
Sample Time
. . . . These timings do not include any distortion in I,he dolo set and
transmission foci lily_
Figure
23.
Character Synchronization
Response. The negative, positive, or no response
at all to addressing or polling.
@
EOA (Pound sign character)
This signal indicates:
• The Bid key has been pressed and the transmis-
sion of text data is to fellow. This pertains to
point-to-point operation
~
the station control
feature installed.
• An end-of-address, with text data to follow.
This
is used in mUltiplexer operations
~
the station
control feature installed. This signal starts the
LRC counters at both the sending and receiving
terl'ninals. The EOA signal is not included in the
following LR C check.
• Positive response to a poll from the multiplexer.
©EOT
This signal indicates:
• An end-of-transmission, and resets the
LRC
counter..:; at both the sending and receivingtermi-
nals.
@EOB
This Signal indicates tht'
.end
of a unit·-block of text.
This is used when the checking feature is installed
and is followed by the
Ll~C
character. This
pl~O\'ides
an
LRC
check comparison at the receiving terminal
with the EOB character included in the check.
®
Positive Respons~~ Yes (Period char~ter)
'\~~.
When this is received, it'indicates:
• A positive response to an address.
• A positive answer to an EOB when the checking
feature is installed.
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