AM3270/TR3271 Reference Manual

Using Multiple-Level Access Methods
A Closer Look at TR3271
086705 Tandem Computers Incorporated 5–11
the host with the text. When all text has been sent or a nonrecoverable error occurs,
TR3271 completes the WRITE request to the application.
TR3271 Product
Environment
Table 5-2 lists some of the features of TR3271 as they apply to Tandem Guardian 90
systems with a 3650/6100 CSS.
Table 5-2. TR3271 Product Environment
TR3271 3650/6100 CSS
Maximum Devices/Line—253* YES
ASCII/EBCDIC Character Code YES
Maximum Line Speed—19.2Kbps YES
Multiple Control Units/Line YES
Printer Support YES
Terminal Screen Sizes—12x40, 24x80, 32x80,
43x80, 12x80
YES
Elect. Interfaces—RS232/V24 YES
Fault Tolerant to Controller YES
Pass-Through to AM3270 YES
EM3270 Support YES
Control Utility SCF
*The figure quoted for maximum number of devices per line is an addressing limitation. The actual
number of devices configured for a line depends on the response time desired, as determined by
factors such as line speed, block size, and transaction rate.
Using Multiple-Level
Access Methods
TR3271 is a layered access method. Actions required to perform specific functions for
the file-system interface, the bisynchronous protocol, or the hardware driver are
performed by various levels within the access method. The I/O interface procedure
(IOP) accepts requests from the file-system and routes them to the appropriate level
for processing. TR3271 provides the following:
Level 1—the bisynchronous driver—interfaces with the hardware controller to set
up and execute operations that result in the transmission or reception of messages
on the communication line.
Level 2—the multipoint tributary supervisor—conforms to the Binary
Synchronous Communication (BSC) protocol. Level 2 protocol is driven by
requests from the request processor (level 3 protocol) and completed events from
the driver (level 1 protocol).
Level 3—the request processor—accepts events from the I/O interface which may
be file requests such as OPEN, READ, and so on, or externally generated actions
such as SCF operations. Level 3 protocol preprocesses these requests and
determines whether the level 2 protocol should be called or the appropriate
subdevice protocol (level 4 protocol) should be invoked to provide further
processing of the request.