IBM 2790, Руководство пользователя, Страница: 33 / 88

after data detection, the command proceeds to its normal
ending point. The ending status will be CE-DE.
Sense. The result of Halt I/O for this command is the same
as for a Read command.
No-Op. Halt I/O will not affect this command since it per­
forms no operation and is handled as “command immedi­
ate.”
Test I/O. Halt I/O will not affect this command since it per­
forms no operation in the 2715 and is handled as a “com­
mand immediate.”
Command N ot in Process. If the 2715 is command-free, the
Halt I/O will not affect the present state and will be disre­
garded. No ending status will be generated for this condi­
tion.
Binary Synchronous Communications Adapter (Standard on
2715 Model 2)
The Binary Synchronous Communications (BSC) adapter
permits remote operation of the 2715 Model 2 over
common-carrier or privately owned communications facili­
ties with: (1) a System/360 equipped with a 2701 SDA-II
or a 2703 Synchronous Attachment; or (2) a System/360
Model 25 equipped with ICA/SDA. Operation is in half­
duplex mode for data, though the transmission facility can
be two-wire or four-wire. The code is EBCDIC. The trans­
parency feature must be specified for the 2701 SDA-II or
the Model 25 SDA. Transparency is standard on the 2703
I with EBCDIC code. The 2715 does not support USASCII
I code because data transmission to and from the System/360
is in transparent mode only.
Communication between the System/360 and the 2715
Model 2 will be via one of the following three configurations:
1
. Point-to-point, non-switched—at 600*, 1200, 2000,
2400, or 4800 bps.
2. Point-to-point, switched—at 600*, 1200 or 2000 bps.
3. Multipoint**, non-switched-at 600*, 1200,2000, or
2400 bps.
BSC functions provided by the 2715 are: identification,
polling, contention, selection, headers (nonuser 2715 to
System/360 data only), transparent text, terminate, and dis­
connect.
If the data set does not provide clocking, a Synchronous
Clock feature is required. Refer to Teleprocessing Systems
Summary , GA24-3090-3 or later edition, for information
on data-set attachments.
*600 bps is for World Trade modems only.
**Multipoint operation requires Expanded Capability feature.
Non-Switched (or switched with network previously established)
Transmitting Station Sends - EN Q DLE STX—text— DLE ETX-------
Listening Station Sends 'ACK0'
or
NAK*
or
W ACK**
Switched (connection not previously established)
Transmitting Station Sends - A EN Q DLE STX—text— DLE ETX------
I |
Listening Station Sends A ACK0
or
A NAK*
or
A WACK**
A — Identification Character(s)
* N A K — Negative, station is not able to receive
**W A CK— Temporarily negative, station not presently able
to receive.
Figure 20. Point-to-Point Line Initialization Examples
Ini tia Hza tion—Po in t- to -Point
When a station (System/360 or 2715) determines that a
transmit queue contains a message to be transmitted, it
enters the initialization stage of transmission. This station
then contends for the line by sending ENQ—or A ENQ
[identification character(s) followed by ENQ character] on
a switched network—to the other station. The listening
station then responds with a positive acknowledgment
ACK0 (or A ACK0 on a switched network), thereby com­
pleting the initialization stage. Figure 20 shows examples.
Ini tializa tion—Multipoin t
In the multipoint operation, the System/360 (control
station) controls all initialization. If the System/360 desires
to transmit a message to the 2715, it will go into selection
mode. If it desires to service the 2715—that is, check to
see if the 2715 has a message(s) to transmit—it will go into
polling mode. The positive response to selection is ACK0,
whereby the System/360 then transmits its data. The posi­
tive response to polling is the text message itself. Figure 21
shows examples.
Text Replies
At the completion of the initialization stage, the data link
is established between the System/360 and the 2715, and
the master starts the transmission of the text message. The
text message is always preceded by the control characters
DLE, STX, and is always followed by the control characters
DLE, ETX (ETB). The slave responds to the ETX (ETB)
character with a positive acknowledgment ACK0 or ACK1
to the master in case of correct transmission, or with the
negative acknowledgment NAK in case of incorrect trans­
mission. Upon receiving a NAK indication, the master will
repeat transmission of the same message until successful
IBM 2715 Transmission Control Unit 33