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NOTE: All 2780 terminals are shipped as primary terminals
(1 second timeout).
Terminals can be altered for secondary
operation by an IBM Customer Engineer at the time of
installation.
One-Second Timeout
This timeout is used when operating point-to-point
on a contention basis.
It
is the period of time that
a primary terminal allows a secondary terminal to
reply to the Enquiry (ENQ) character. The primary
terminal automatically retransmits the ENQ char-
acter after the one-second timeout.
Two-Secono Timpont
A receiving terminal must respond to a block-
checking operation within two seconds.
If
unable to
do so, the receiving terminal remains in receive
mode and waits for the transmitting terminal to send
an ENQ character to solicit the response.
Three-Second Timeout
This timeout is used when operating point-to-point
on a contention basis.
It
is the period of time that
a secondary terminal allows a primary terminal to
reply to an ENQ character.
At the end of this time-
out the ENQ character is automatically encoded again
and sent to the primary terminal.
A transmitting terminal will wait three seconds
for a response to a block-checking operation.
If
a
reply is not received, an ENQ is automatically
encoded and sent to the receiving terminal to solicit
the response.
A receiving terminal initiates a three-second
timeout upon receiving a sync pattern (SYN SYN).
It
must receive an STX character or another sync pat-
tern within this time.
If
none is received, the termi-
nal abandons synchronization and waits for another
sync pattern. Upon receiving an STX character,
another three-second timeout is initiated.
If
a block-
end character or a sync pattern is not received within
this time, synchronization is abandoned.
When operating terminal-to-terminal over leased
lines, one terminal must be designatea the primary
terminal and the other the secondary terminal. This
enables the primary terminal to gain control of the
line
if
both terminals bid for the line simultaneously.
This designation is not necessary when operating
terminal-to-terminal over switched (dial) line facili-
ties, since the operators control the priority of the
terminals. In a terminal-to-CPU (contention mode)
operation, the 2780 is always the primary terminal.
38
The CPU will wait for the 2780 to send the second
ENQ character if the CPU receives an ENQ response
to its ENQ character. This condition could exist
if
both the 2780 and the CPU send ENQ characters at
approximately the same time.
If,
in a terminal-to-CPU operation, the CPU
transmits for longer than one second without a
turnaround, the CPU must send sync patterns at one-
second intervals.
Extended ENQ Retry Feature (48-Second Timeout)
The Extended Enquiry (ENQ) Retry Transmission
feature is required only when 2780 terminals are
used with a System/360 that is processing data
transmitted over many communications lines. Under
these circumstances the CPU may experience
difficulty in responding to a specific 2780 communi-
cations line within the present 12-second timeout
limitation. The availability of this feature is
limited to 2780 Modell and 2 terminals using the
EBCDIC code.
Standard 2780 Operation
A block check response other than D LE 0, D LE 1,
NAK, EOT, or RVI causes the transmitting 2780 to
send an ENQ (Enquiry) character. The ENQ char-
acter is also transmitted if the transmitting 2780
receives no response at all within three seconds.
The ENQ character serves as a request to the
receiving CPU to repeat the last block check re-
sponse. A maximum of three retry ENQ characters
are transmitted by a standard 2780 terminal. Fail-
ure to receive a recognized response after three
ENQ's causes the transmitting 2780 to send an EaT
character and time out. The EaT character is
transmitted three seconds after the last ENQ is
transmitted.
2780 Operation with Extended Enquiry Retry Trans-
mission Feature Installed
The purpose of the Extended Enquiry (ENQ) Retry
Transmission feature is to allow the transmitting
2780 to count up to a maximum of 15 retry ENQ's in
an effort to recognize a response prior to sending
an EOT character and timing-out with an error con-
dition. Since three seconds elapse between each re-
try, and also between the last ENQ and EaT charac-
ter, a total of forty eight seconds elapse before an
EOT character is transmitted ending the transmission.
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