AM3270/TR3271 Reference Manual

Bisync Procedures
Bisync Protocols—Concepts and Terminology
086705 Tandem Computers Incorporated 1–11
The polled 3270 CU remains selected at the completion of a poll operation so that
the application can issue a WRITE, ERASE/WRITE, ERASE/WRITE
ALTERNATE, COPY, or EAU command without having to reselect the 3270 CU
and its subdevice. (These refer to commands sent inside the data stream, rather
than to Guardian 90-specific commands.)
To terminate the general poll, an RVI issued to the 3270 CU forces an EOT
response. Alternately, a response other than the ACK that is expected after blocks
ending with ETX, will terminate the general poll. The receipt of an EOT signaling
the end of transmission (or nothing to send) will also terminate a general poll.
2. The specific poll:
When the supervisory station wants to contact a specific tributary, it sends a
specific polling sequence to an IBM 3270 CU and one of its subdevices. The
subdevice is identified by the specific device address in the specific poll sequence.
If the polled station has something to send, it sends the message in a data block. If
not, it responds with a nothing-to-send in the form of an EOT.
3. The select:
When the supervisory station has something to send, it first sends a selection
addressing sequence addressed to that particular station. If the selected terminal
is available to receive, it responds with a ready-to-receive (ACK 0). If it is not
available , it responds with a WACK or RVI.
The selection addressing sequence specifies, for example, a 3270 CU and one of its
attached subdevices. The sequence is similar in format to a specific poll sequence,
but the selection addressing sequence is completed using different hex codes for
the address.
The 3271 and 3275 CU return an ACK0 to the supervisory station if the selection
and buffer transfer between the CU and the device were successfully completed.
When a 3275 is to be selected, the device number 0 is always addressed.
The 3274 and 3276 CU provide a positive response to a selection sequence before
transfer of a device buffer to the 3274 or 3276 occurs. If an error occurs during
buffer transfer when it follows receipt of a valid selection addressing sequence, a
positive response to the selection sequence is sent by the 3274 or 3276, and
status/sense bytes are set. An EOT is sent in response to the next 3270 command.
The Role of Cluster Controllers and Front-End Processors
The type of general polling described in the preceding paragraphs is sometimes called
“roll-call” polling, and, in large networks, the response time can be lengthy. To
overcome these delays, multipoint links often use cluster controllers and front-end
processors.