AM3270/TR3271 Reference Manual

Using Multiple-Level Access Methods
A Closer Look at TR3271
086705 Tandem Computers Incorporated 5–13
Level 2 Protocol Level 2 provides the operations required to sequence a communication line through
the bisynchronous multipoint tributary protocol. Since the host controls the line,
requests from applications do not have any direct effect on line state. Data may be
written to the host only when the host polls the line. READ requests are satisfied
when the host chooses to select the subdevice and send data to it.
ASCII/EBCDIC Line Code
An application process need deal only with the ASCII character set. If the line code is
EBCDIC, the synchronous controller is configured during system generation to
automatically translate from ASCII to EBCDIC.
Retries
Line operations are subject to transmission errors. Errors may be due to various
causes, such as BCC errors, a nonresponding host, or incorrect responses from the
host. Generally, when errors of these types are encountered, TR3271 retries the
requested operation.
A retry-count value is specified for each communications line at system generation.
This count determines the maximum number of times that TR3271 will retry an
operation when an error condition is encountered. If an error condition persists after
the operation has been retried the specified number of times, an error indication is
returned to the application process that requested the operation.
Timeouts
Timeouts are used within the bisynchronous protocol for error detection. Each
communication line has a timeout value that is specified at system generation. This
value should be specified as a value greater than 2.7 seconds, or less than or equal to 3
seconds.
TR3271 uses the timeout value to determine whether the host is responding to
transmissions which expect a response. For example, text transmitted to the host in
response to a poll awaits an acknowledgment from the host before continuing.
Level 3 Protocol Level 3, the request processor, provides the interface path between the I/O interface
procedure requests and levels 2 and 4. Level 3 preprocesses all requests and decides
which level will perform the initial or the continued action. The subdevice (level 4)
protocols are called directly by level 3. Level 4 returns results to level 3 and level 3 in
turn may either return immediately to IOP or call level 2 to initiate a line operation.
Resource Management
For certain physical I/O operations performed by the driver, TR3271 uses a dedicated
buffer (unnamed) whose size is specified at system generation. These operations are:
receiving control sequences such as polls and selects from the host, transmitting status
messages and control sequences to the host, and receiving responses to WRITEs.
When the host polls a subdevice that has a WRITE request pending, TR3271 requests a
buffer that is large enough to accommodate the WRITE count specified in the request