AM3270/TR3271 Reference Manual

AM3270 Product Environment
A Closer Look at AM3270
3–10 086705 Tandem Computers Incorporated
ASCII/EBCDIC Line Code
An application process need deal only with the ASCII character set. For example, if
the line code is EBCDIC, the CLIP in a 3650/6100 CSS can be configured through SCF
to automatically translate from ASCII to EBCDIC (and the reverse). This can also be
accomplished through system generation parameters.
Retries
Line operations are subject to transmission errors in the form of BCC errors,
nonresponding remote stations, and incorrect responses from remote stations.
Generally, when errors of this type are encountered, AM3270 retries the requested
operation.
A retry count is specified for each line at system generation. The retry count value
determines the maximum number of times that AM3270 retries an operation that
encountered an error. 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 line
has a timeout value specified at system generation. The timeout value should always
be specified as a value greater than 2.6 seconds and less than 3 seconds. (Standard
bisync protocol has a timeout value of 3 seconds.)
During a call to the WRITE procedure, the timeout value determines the period of time
after a message is sent to a subdevice that AM3270 waits for an acknowledgment.
During a call to the READ procedure, the timeout period determines how long
AM3270 waits for either an EOT from a polled controller or a message from a
subdevice that responds to a poll.
Polling operations are retried a maximum of the number of times specified for retry
count before an error indication is returned to the application process.
Level 3—Request
Processor
Level 3, the request processor, provides the interface path between I/O process
requests and levels 2 and 4. Level 3 preprocesses all requests and decides what level
will perform the initial or continued action.
The subdevice protocols (level 4) are called directly by level 3. Level 4 returns results
to level 3. Level 3 then either completes the action immediately or performs a call to
level 2 for initiation.
Level 4—Subdevice
Protocols
A subdevice attached to an AM3270 line represents the logical entity that receives file-
system requests from an application process. A one-to-one relationship exists between
a subdevice known to AM3270 and a physical device attached to a cluster controller.
A subdevice is added to an AM3270 line through SCF. Associated with each
subdevice are attributes that define the operating mode. Subdevices are referenced by
application processes through a standard Tandem file name.