IBM 2790, Manual

Page: 34 / 88

Multipoint-Selection
System/360 Sends - EOT A ENQ, DLE STX— text— DLE ETX-
2715 Sends k C K cl
or
A is a selection address. N A K *
or
W A CK ***
Multipoint-Polling
System/360 Sends - EO T A E N Q
2715 Sends !DLE STX— text— DLE ETX
or
A is a polling address. EO T**
* N A K — N egative, 2715 is not able to receive, which implies that
operator intervention is required.
**E O T — N egative, 2715 has no data to transmit.
*** W A C K — Temporarily negative, station not presently
able receive.
# Figure 21. Multipoint Line Initialization Examples
transmission is achieved or until the number of retries has
been exhausted. After receiving a positive acknowledgment,
the master continues to transmit its text messages, with the
same format just described, until the transmit queue has beer
exhausted. If the master receives an improper ACK (that is,
ACKO when ACK1 is expected or ACK1 when ACKO is
expected, after a text message but before having sent ENQ),
the master will transmit ENQ a specified number of times
until a negative acknowledgment or the proper response has
been received. If the proper acknowledgment is received,
the next message is transmitted; if a negative acknowledg­
ment (NAK) is received, the message is retransmitted.
If no response (or an invalid response) is received to the
message transmitted, the master will transmit ENQ until
either a response is received or the specified number of
retries has been exhausted. If the proper response is received
the next message is transmitted. If an improper acknowledg­
ment is received, the last message is retransmitted. (If the
2715 is the master station and the error-retry count is
exhausted, an operator-awareness message “data link not
operational” is sent to the 2740, and the 2715 rebids for
the line.)
Switched Network Operation
During the initial configuration of the system, the 2715 can
be assigned to be either an originating or an answering station.
If assigned as an originating station, the 2715 continuously
monitors ‘data set ready’. When it detects that the data set
is ready, the 2715 sends an identification sequence followed
by ENQ. When it receives a positive acknowledgment from
the CPU, the 2715 sends data, if available, or EOT if no data
is available.
If assigned as an answering station, the 2715 can operate
in either manual or auto-answer mode, depending on the
data-set hookup. When the 2715 is an answering station and
its data set receives a signal, the 2715 monitors for an identi­
fication sequence. If no identification sequence is received
within
20
seconds, the 2715 disables the line (timeout con­
dition). If the identification sequence is received, data
transmission is maintained as long as there is data to trans­
mit. The CPU can either disconnect the line at the end of a
completed transaction by sending DLE EOT, or keep the
line connected by sending EOT. When the CPU sends EOT,
the 2715 sends either another message (if there is another
message in its queue) or EOT. It is up to the CPU to dis­
connect the line.
The 2715 disables the data set (“ go-on-hook” ) when
DLE EOT is detected or when no activity is present on the
line for
20
seconds.
Special BSC Considerations for the 2715
Reverse Interrupt (R VI). Reverse interrupt allows a slave
station to request control of the line from the master station.
When the slave station desires to interrupt the master station
to gain line control, the slave station sends RVI instead of
a normal positive acknowledgment. If the 2715 receives the
RVI, the 2715 responds with EOT providing it has more
than 896 bytes of core storage available for data. If the
2715 is the slave station, it transmits RVI when it has a
queued, priority message.
Wait Before Transmit-Positive Acknowledge (WACK). The
WACK character is a positive response from the slave station
that requests the master station to wait before it transmits
further text messages.
When the 2715 is the slave station, it responds with
WACK when it has more than 504 bytes (for 16K storage)
or 1134 bytes (for 32K storage) of data from the CPU that
is waiting to be transferred to the 2740, a 1053, or main­
tenance module.
When the 2715 is the master station, a response of WACK
causes the 2715 to continue to send ENQ’s until either the
CPU responds with ACK or a user-specified number of
ENQ’s has been transmitted. If the CPU responds with ACK,
the 2715 sends the text message. If the specified number
of ENQ’s occurs before an ACK is received from the CPU,
the 2715 sends EOT in response to WACK.
Termination (EOT). When the master station terminates
message transfer and the appropriate positive acknowledg­
ment is received in response to its last message, the master
will transmit EOT to the slave. The EOT will put the link
in control mode, ready for new transmissions. Thus, the
transfer state is terminated.
The 2715 transmits EOT to indicate some unusual con­
ditions as follows:
• EOT is transmitted when the sum of message-transaction
lengths plus two does not equal the total message length.
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