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To Computer
Mul tiplexer
Terminal Aaaress _ _ A Soace
Group
ACldfe\s
_ _
G
Space
All Coli
Address _ _
Spoce
B
Space
G
Space
Space
C Space
H
Space
Space
D
Space
H Space
Space
Gro",~
Maste
Ali Call Mastel
Figure :32. Station Control Addresses
the multiplexer either selects the terminals to re-
cei\"(~
a message or it polls a terminal gh'ing
it
an
opportunit~
to transmit to the multiplexer.
The first character of the address is a letter from
A to Z. Each letter selects a terminal (Figure 32).
Some letters can select a group of terminals to re-
cei ye
a
message from the multiplexer.
However, a
group address cannot be used to poll a group of ter-
minals because contention would result. The first
character can also be a slash (/) which selects all
terminals on the line. This is called broadcasting.
The second eharacter of the address is a space.
Multiplexer
©
®
NON-
SWITCHED
LINE
Ter.minal
A
D
o
S
R
P
E
A
s
C
S
E
*
If
@
response, the multiplexer will not follow with
®
and text but will repeat address or address some othe.r terminal.
CY)*
Control Characters
In addition to the line control characters used for the
basic terminal, the
®
(comma, A821C) identifies
an addressing operation.
Operation
For terminals to communicate with the computer
without contention (more than one terminal attempting
to use the communication line at the same time), a
special addressing andPQlling system is used.
Addressing is the preparing of a group
of
termi-
nals(or one terminal) to receive data.
Polling is the informing of an individual terminal
that it and
it
alone may transmit data to the multi-
plexer. Terminals are polled in a predetermined
order controlled by the computer program. An opera-
tor who wishes to transmit presses the bit
key
to sig-
nal the multiplexer.
Polling places the terminal in
transmit text mode and the operator can then send the
message.
Both individual terminal addresses and group ad-
dresses are wired in the terminal; the all call address
is not wired. When either a group or all call address
is used, one terminal
in
the group is assigned the role
of master (all others being subordinate). For the
all call address, only one terminal on the communi-
cation line is designated as master. For the group
address, one terminal in each group is designated as
master. This designation is made by jumper wiring
within the terminal. These master terminals re-
spond to the addressing by informing the multiplexer
@
Text
©
Figure .33. Station Control, Multiplexer Addressing (without Checking Special· Feature)
• 28
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