AM3270/TR3271 Reference Manual

5 A Closer Look at TR3271
086705 Tandem Computers Incorporated 5–1
This section includes:
An overview of the TR3271 communications subsystem
Examples of TR3271 process configuration
Details about subdevice access, configuration, and control
A list and explanation of the level 4 (subdevice) protocols
TR3271-specific operations and file system procedures
A list and details about TR3271’s four process levels
Overview The TR3271 Tributary Access Method provides communications with IBM mainframes
where the Tandem system appears to be one or more 3270 terminals connected to one
or more byte-synchronous 3271 cluster controllers. Under this access method, one
data-communications process controls a single byte-synchronous line.
TR3271 simplifies the tasks associated with connection to a tributary (polled) line from
an application process. Application processes OPEN, then READ and/or WRITE on
subdevices that do not physically exist.
TR3271 is responsible for satisfying the host’s terminal addressing requirements.
TR3271 performs this function by mapping device addresses into the types of
messages required by the host. Additionally, TR3271 queues outbound messages and
status information, responds properly to host polls, and delivers incoming messages
from the host to the proper processes.
TR3271 also provides a pass-through function that is used in conjunction with
AM3270. With the pass-through function, data from AM3270 terminals is passed to
the host through TR3271 and conversely, data from the host (for 3270 terminals) is
passed by TR3271 to AM3270 without the necessity for an intervening application
program.
Three TR3271 Modes TR3271 may be used in three different ways:
With application programs acting as one or more 327x terminals or 328x printers.
Pure pass-through—Subdevices use the pass-through-protocol (PTP), which
logically connects them with AM3270 subdevices.
Programmed pass-through—Application program(s) can open both TR3271 and
AM3270 devices and can perform routing functions, depending on data input at
the terminal.
When used together with the AM3270 pass-through protocol TR3271 allows actual
3270 devices to:
Connect to a Tandem system.
Access application programs running on an IBM system connected to the Tandem
system.