AM3270/TR3271 Reference Manual

TR3271-Specific Operations
A Closer Look at TR3271
086705 Tandem Computers Incorporated 5–5
Data Buffers
TR3271 does not checkpoint data buffers. For example, if data is received and
acknowledged, and the TR3271’s processor fails before the data can be transferred to
the application process, the data will be lost.
Multiple-Address Versus
Single-Address Lines
In the 3650/6100 CSS environment, the monitoring of subdevice addresses is a
function that is handled within the CSS, thereby removing the task from the Tandem
CPU. This section describes how the 3650/6100 CSS and the TR3271 process interact
in that situation.
A 3270 terminal is addressed by specifying the cluster controller address followed by
the address of the terminal. Each Tandem controller port can monitor a pair of
addresses, one for poll and one for select. This means the Tandem controller can
monitor for a poll or select to any emulated terminal or printer on an emulated cluster
controller.
If polls and selects for other cluster controllers arrive when the Tandem controller is in
single-address mode, the controller ignores them and continues searching for one of its
own addresses. When TR3271 uses this feature, it acts as one cluster controller whose
address pair is specified at system generation.
Multiple-Address Caveat
TR3271 can also simulate multiple cluster controllers; in this case, no address is
specified during system generation. When the multiple-address feature is used, the
port involved must be on a unique line from the host; that is, not on a multidrop line
with other TR3271’s or with other simulated or real cluster controllers. (If the port is
not on a unique line from the host, much CPU time could be wasted in fielding
undesired data.)
Even though this recommendation is not mandatory, it is an absolute requirement that
control unit addresses not be shared with any other devices, such as other Tandem
ports or real cluster controllers on the same line. Shared control unit addresses would
result in more than one unit answering the same polls and selects.
Priming Read A host system can issue a poll or select to an emulated subdevice that has been added
to the system, but has not yet been put into use. TR3271 monitors polls and selects
meant for subdevices in this situation and is responsible for issuing an appropriate
response.
Appropriate Poll response
Before an application can use an emulated terminal, add the emulated terminal using
the SCF ADD command. Similarly, add the emulated terminal before an SCF
CONNECT command can be issued to establish a link to AM3270.
A host could poll or select an emulated terminal (or its cluster controller) before the
terminal is in use. That is, the host could be trying to communicate with the emulated
terminal after it has been ADDed, but before an application OPEN or an SCF
CONNECT has put the terminal in use.