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MULTIPOINT LINE CONTROL
This special feature allows a number of 2780 termi-
nals to operate with a CPU over a nonswitched com-
munications line. The CPU is the centralized master
terminal and controls the operation of each 2780
terminal. A polling or selection operation is initia-
ted when the master terminal (CPU) transmits a
three-character sequence; the first character is
terminal address, the second character is compo-
nent selection, and the third character is an ENQ
that ends the addressing sequence. The following
component-assignment characters are used:
Character
3
4
6
Component
Printer
Punch
Reader
Any alphabetic character (either upper or lower
case) can be specified for a 2780 terminal address,
and is set by an IBM Customer Engineer at the time
of installation.
No general (broadcast) polling character or selec-
tion character is provided with this feature. The
desired terminal and component must be selected
and the polling or addressing sequence is recognized
only when the characters are contiguous, for example
EOT PAD SYN SYN A3 ENQ. The PAD character
is not required when using Six-Bit Transcode.
A three-character sequence using a 3 or 4 for com-
ponent assignment is called a selection sequence. A
three-character sequence using a
6
for component
assignment is called a polling sequence. The poll-
ing and selection format follows:
Polling
POLLING
CPU
POLLED
TERMINAL
E
E
o
A6 N
T
Q
D
Ll
E
S
Ebb
T Text 1 T c c
X
B c c
S
T Text 2 ...
X
SPECIAL FEATURES
Responses to Polling
E
O--Terminal not ready to transmit.
T
S
T--Positive response, data follows
if
the
X
terminal is ready to transmit,
A CPU timeout will occur if the polled terminal is
off-line.
Selection
E
E S
Ebb
E
E S
SELECT-
o
A3 N T Text Tcc
o
B3 N T Text ...
ING CPU
T
Q X
Xc c
T
Q X
D
D
D
SELECTED
LO
Ll
LO
TERMINAL
E
E
E
Responses to Selection
D
LO--Positive response, selected component is
E
ready to receive.
N
A--Negative response, selected component not
K
ready to receive.
A CPU timeout will occur
if
the selected terminal is
off-line.
When the Multipoint Line Control special feature
is used, the basic component-selection feature (ESC
plus follower code to select the punch or printer) is
inoperative. Selection is controlled by the address
sequence of the Multipoint feature only. However,
ESC sequences can still be used for vertical forms
or horizontal-format-control.
Special Features
47
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