AM3270/TR3271 Reference Manual

Bisync Procedures
Bisync Protocols—Concepts and Terminology
1–10 086705 Tandem Computers Incorporated
ENQ—Used to indicate both poll and select. The type of request is determined by
the encoding of the address characters.
ACK and NAK—Used in two ways: In response to a previously transmitted data
block, and as a response to a select request (ACK0 only). NAK applies to a poll
but not to a select.
EOT—Used in two ways: As a nothing-to-send response from a polled station,
and to indicate the end of a complete message exchange (single-block or
multiblock), which clears the link for further transmission.
Polling and Selecting Compared
The supervisory station determines which station is currently linked, and which
direction the transmission is going:
The poll is an invitation-to-send sent from the supervisory station to the tributary.
The tributary station can respond to the supervisory station with a message or
status.
The select is a request-to-send (transmit) notice sent from the supervisory station to
the tributary, telling the tributary station that the supervisory station has
something to send.
The following list compares the basic steps in a general poll and a specific poll. Also
included is a brief description of the variations among some common IBM terminal
control units (TCUs) and control units (CUs).
For a detailed discussion of CU sense and status bytes (which can be displayed by
using the SCF command STATUS SU), including a table that lists sense/status bytes,
refer to the SCF Reference Manual for AM3270 and TR3271.
1. The general poll:
At periodic intervals the supervisory station, which may be a CPU or a front-end-
processor, sends a general polling sequence, which invites any attached station to
send a message, if it has one waiting. A poll may also be sent to each CU, such as
an IBM 3271, 3275, or 3276. The CU then examines each subdevice attached to it,
in sequence, starting at a random device address.
When the CU is a 3275, it responds as if it were a 3271 or 3274 with one subdevice
attached. When the CU is an IBM 3274 or equivalent, each attached subdevice is
examined in the order in which the Enter key was pressed on the subdevice.
If a message from a CU’s subdevice is waiting, it is transferred to the CU, which
then sends it on to the supervisory station. The message includes the address of
the originating subdevice. Upon completion of the transfer, an ACK response
from the application causes the 3270 CU to continue its response to the general
poll , either by transferring another block of text from that subdevice, or by
responding with an EOT to reset the line to the control state, ending the
subdevice’s reply.