Table of Contents
Advertisement
the terminal called will initiate a disconnect se-
quence
if
its reader is not ready. If the reader is
ready at the called terminal, the terminal will
initiate a Line Bid sequence after the 20-second
timeout has occurred. If a positive response to
this Line Bid is not received, the audible alarm will
turn on and a second 20-second timeout is started.
If this timeout is completed before a character is
received, the called terminal will disconnect. This
prevents the called terminal from being left off-hook
(left unterminated) by a call that is not intended for
it (wrong number), by a system failure, or by the
completion of normal transmission.
This is not an
error condition and is not indicated as such.
After the transmission has been completed, the
originating terminal
(if
a CPU) ends the call by send-
ing a DLE EOT, which causes the called terminal to
disconnect automatically. If the originating terminal
fails to transmit a record correctly within the allot-
ted number of retransmissions (3), the originating
terminal should end the operation. The normal dis-
connect sequence is as follows when a 2780 is com-
municating with a CPU:
D
D E
CALLING CPU
LO
L
0
E
E T
E
E
CALLED TER M ... Text T
0
X
T
or:
CALLING CPU ... Text
CALLED TERM
E
D E
T
L 0
X
E T
D
LO
E
When operating point-to-point (2780 to 2780), the
auto-answer terminal will disconnect automatically
after receiving the EOT if there is no subsequent
transmission within 20 seconds.
so
NOTE: When switching from receive to transmit mode on a
2780 terminal equipped with the Auto Answer feature and
operating over a switched network, the operator must first
press the Auto Answer key on the 2780 (turns Auto Answer
feature off) prior to changing the Mode switch from Receive
to Transmit. If this procedure is not followed, a power-on
reset occurs which causes the data-set-ready condition to
drop out. However, if the data-set -ready condition
is
dropped, it can be re-established by: (1) pressing the Auto
Answer Key on the 2780 (if not already operated, turns Auto
Answer feature off), (2) removing the receiver from the data
set, (3) pressing the Talk button on data set, (4) pressing
Data button on data set, and (5) replacing receiver.
The
data-set-ready condition will be re-established provided the
calling station has not been disconnected.
MULTIPLE-RECORD TRANSMISSION
This special feature allows up to seven records to be
transmitted before a line turnaround occurs. How-
ever, if the 2780 is operating at 4800 bps line speed,
no additional throughput can be expected when this
feature is used. The specific number of records
depends upon the length of each record (with a max-
imum total of 400 characters). If less than 80 char-
acters are to be read, the record length is defined
by use of the EM (End of Media) character. The US,
ETB, or ETX control character will be transmitted
following the transmission of the EM character.
Full 80-character records will generate a US, ETB
or ETX control character automatically.
The 400-character limitation includes all data;
all component selection, vertical forms control,
and horizontal forms control sequences; and all
end-of-record characters--but it does not include
the STX character that the 2780 encodes when
transmitting. When the EBCDIC code is used, the
transmitting and receiving terminal will consider
the modified LRC check characters as part of the
400-character limitation. One modified LRC check
character is generated for each record.
Line buffer overrun will not occur until more than
seven records per block are received or the 400-
character limitation is exceeded, when this feature
is installed. The operation of the line and I/O buf-
fer for handling a data record is not changed by this
feature. The)nput and output counter capacity is
increased to provide 7 record (maximum) capability.
The Multiple-Record Transmission feature
operates in the following manner: Assume that the
cards to be read are punched in all 80 columns.
Cards 1 through 5 will be read and the US character
will be generated automatically at the end of each
card read.
• Card 1 is transmitted while card 2 is read and
then transferred from the I/O buffer to the line
buffer.
• After card 5 has been read, the 2780 machine
logic determines that the data read cannot be
accommodated by the 400- character buffer
capacity; the US character generated at the end
of card 4 is therefore retranslated to an ETB and
transmitted. The ETB initiates a block check
response, which causes a line turnaround from
the receiver. Overrun prevents the counter from
increasing to 5. No error condition exists. A
stop-load condition exists, howe ver, and causes
the reading of cards to be momentarily suspended.
Manual inte rvention by the operator is not re-
quired under these circumstances.
Advertisement
Table of Contents
