Communications Control Characters - IBM 5100 Maintenance Information Manual

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COMMUNICATIONS CONTROL CHARACTERS
~ages
are
indicated~
two line control characters:
®
(circle
D)
and \.S)(circle C). A
@
~ates
the
start of a message, or start of transmission; a
~indi­
cates the end of a message, or end of transmission. These
characters are transmitted by the 5100 and by the remote
system as shown in the following diagram (the arrow indi-
cates the direction of data flow):
Action
The lines 'clear to send' and 'data set rdy'
are activated by the data set, which is
indicated by the arrow on the status line
of the display screen pointing up
t.
IBM 5100 Remote System
The 5100 waits approximately 1 second
after 'clear to send' and 'data set rdy'
become active, then sends the first mes-
sage to the remote syste m.
© ...
data@l
>
- = - - - - - = - -
The 5100 waits for the remote system to
send a message.
The remote system responds.
<,..--_@=--D_dat_a ..
_.©=_
After a 66-millisecond delay, the 5100
changes to the transmit state, and trans-
mits the next message.
© ...
data@
>
~-------,
1
The
®
is transmitted by the communications pro-
gram immediately after the approximate 1-second delay,
which is indicated by the arrow on the status line chang-
ing from up (t) to right
(~).
If the remote system does not respond to a message from
the 5100 or if one of the control characters is lost (be-
cause of line noise or some other reason) the 5100:
• Remains in the receive state until a
©
or
@
is
received.
•
Ign~res
all data characters until a
©
or a
®
is
received.
•
Re~s
in the receive state if a
@
is received but
no
~
is received. (Any data received is stored in
the extended display but is not printed or written on
tape.) The 5100 leaves the receive state when either the
ATTN key is pressed (which transmits a receive inter-
rupt to the remote system) or the CM D and A TIN
keys are pressed (which causes the 5100 to go to the
transmit state and send a
@ ).
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- • Goes to the transmit state when a
©
is received be-
" fore a
@.
Any data characters received before
the
©
is received are lost.
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Theory Of Operation 2-7