AM3270/TR3271 Reference Manual

TR3271-Specific Operations
A Closer Look at TR3271
5–4 086705 Tandem Computers Incorporated
Implementation Caveats In a network environment it is not feasible to implement a full software simulation of a
3270-family controller and its devices; therefore, TR3271 is more loosely coupled with
its emulated devices.
Responding to an Application’s READ
Due to this loose coupling, TR3271 does not wait until the application’s READ has
completed before returning the appropriate response (such as ACK) to the host. The
READ, or two-step READ forerunner (CONTROL 22 request) isused by the
application is implicit license to TR3271 to accept and acknowledge data on the
application’s behalf.
READ BUFFER and COPY
TR3271 does not validate outgoing buffer contents, but does check incoming data for
command validity. The READ-type 3270 command, READ BUFFER, is supported as
of the D-series release.
COPY is valid only if either:
A single application process sends an OPEN to both the “to” and “from” devices
exclusively; it has issued appropriate SETMODEs to enable this feature for both
subdevices; and both subdevices are on the same logical control unit, or
Both subdevices are in pass-through mode to the same AM3270 process and are
attached to the same control unit at the AM3270 end.
Responding With an ACK
As noted above, TR3271 will respond with an ACK to an incoming message for a
subdevice that is being read, if:
the block check is good, and
the command contained in the message is valid.
Control sequences are handled in accordance with 3270 protocol. (For information
regarding the IBM family of devices, see the publications listed in Section 1.
Opening Files
IBM 3270 systems treat the 3271 controller as a bidirectional pathway; that is, terminals
may be read or written to at any time. TR3271 provides this capability in that the
application may either OPEN a file with a nowait depth greater than 1 and use
SETMODE 30 (complete in any order), or OPEN the subdevice more than once.
The application must be able to control any confusion that may result from a condition
caused, for example, by the following sequence of events: the application issues a
READ, and it issues a WRITE that is queued by TR3271 (the WRITE and READ may be
issued in either order), then the READ completes before the WRITE (which may refer
to a previous screen) completes.