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lines when using Telegraph Terminal Control Type I
(only). That is, a selected (polled) teletypewriter
station may address another teletypewriter station
on the same line without going through the 2702/03
processor and without causing channel interference
due to data servicing during the body of the message.
The Search command is valid only for Telegraph
Terminal Control Type I lines.
On acceptance of Search, the 2702/03 scans the
first two non-letters characters received from the
line.
If
the first such character received is V (the
negative response to polling for AT&T 83B2/83B3
and W. E. Plan 115 stations), the V is transferred to
the
I/o
channel. The command ends with Channel
End and Device End status.
If
the first two such
characters comprise the sequence AZ (the proces-
sor's own address), then AZ is transferred to the
I/o
channel and the command ends immediately with
Channel End and Device End status.
If
any other non-letters character is received at
the beginning of the message, the 2702/03 monitors
the received data for the FIGS-H, LTRS sequence
(EOT).
If
this sequence is received, the H character
is transferred to the processor, Timeout Sequence
is set, and the command ends with Channel End,
Device End, and Unit Exception status.
NOTE: If the first non-letters character is A, but Z does not
follow immediately because anflther station is being addressed,
the A is transferred to the I/O channel.
Search (2703 BSC with Station Selection Feature)
The Search command is used when the 2703 is a non-
centralized control station in a multipoint network
to monitor the communications line and transfer
received characters to the channel. The Search
command is always chained to a Poll command. The
Search command is normally ended:
1.
When the 2703 is addressed by the polled ter-
minal (SYN SYN Address ENQ). The command
is ended after the ENQ character is received.
2. When an EOT character is received if the 2703
was not addressed by the polled terminal.
The
command is ended after the EOT character is
transferred to the channel.
If
the 2703 is not addressed (terminal address was
received) the message received from the polled ter-
minal is transferred to the channel until an SOH or
STX character is received (this character is last
character transferred to the channel until an EOT
character is received). The 2703 continues moni-
toring the line for an EOT character. The Search
command is then ended after the EOT character is
transferred to the channel.
Adprep (2703 BSC with Station Selection Feature)
The Adprep command is used when the 2703 is a
tributary station in a multipoint network to monitor
the communications line. When the first non-SYN
character recei ved after character phase is estab-
lished is the 2703's group, poll, or selection address,
the Adprep command is ended.
This message is
then transferred to the channel by a chained Read
command.
If
the first non-SYN character received
is not the 2703's address, the 2703 continues moni-
toring for an End character.
If
an SOH or STX
character is received, the 2703 stops monitoring
for its address until a SYN SYN EOT sequence is
received.
IBM TERMINAL CONTROL TYPE I--OPERATIONS
The IBM Terminal Control Type I is designed to
service the following:
IBM 1050 Data Communications System
IBM 1060 Data Communications System
IBM 1070 Process Control System
IBM 2740/41 Communication Terminals
Transmission Code
BCD plus parity is used between the IBM 2702/03
equipped with the IBM Terminal Control Type I and
the 1050, 1060, and 1070 systems or 2740 and 2741
terminals.
Figure 2-20 shows an example of a
character in this transmission code. The start of
a character is always signaled by a start bit (Space
level). The end of a character is always signaled
by a stop bit (Mark level). The start and stop bits
are added to the character by the terminal control
when the 2702/03 is transmitting; these bits are
deleted when the 2702/03 is receiving.
Each received and transmitted character is
checked for odd vertical parity. VRC (vertical
redundancy check) errors caUSe Data Check to be
set in the Sense byte but do not cause termination
of the command currently being executed.
Transmission Code Converted to Interface Code
Figure 2-21 shows the conversion between the I/O
interface code and the transmission code. Interface
bits 1-7 are transmitted as B-C bits. Interface byte
position 0 signals upper/lower case (shift) a 0 (no
Operations
2-39
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