CP6100 I/O Process Programming Manual

Using CP6100: Programming
If no path exists between the controllers and the 6100 subsystem
cabinet--for instance, if both cables are unplugged or the
subsystem cabinet has lost power--CP6100 queues user requests and
completes them every 30 seconds with file system error 231. When
power returns, applications can continue using the line. (If the
SYSGEN AUTOCLOSE parameter is set, applications must close and
reopen the line before they can use it.)
If an LIU does not respond to a request--for instance, if the LIU
is defective--CP6100 tries up to three times to establish contact
through the CSM. In the meantime, requests complete with error
124. If the LIU fails to respond after three attempts at
recovery, CP6100 declares the line down and completes new
requests with error 66. (Then an operator must use CMI to
download the line and return it to service.) If on the other
hand the LIU recovers, applications can continue using the line,
though if AUTOCLOSE is set, they must close and reopen it.
CP6100 also tries to recover if a status probe reveals an LIU
failure. For example, if the probe reports that power has
returned, implying an earlier failure, CP6100 downloads the LIU.
If the SYSGEN AUTOLOAD parameter is set, other problems revealed
by a status probe will also cause a download. Applications can
resume their use of the line, though if AUTOCLOSE is set, they
must close and reopen it.
In none of these cases is the failure transparent to the
application. Even if AUTOCLOSE is not in effect, delays may
cause a timeout or loss of the communication link. Also, even if
AUTOLOAD is set, a power-on condition will be perceived only if a
status probe has occurred (i.e., if the LIU hasn't transferred
data within 10 seconds but has not been without power long enough
to be declared down). To be safe, the operator must restart the
line, and applications must perform end-to-end recovery. If the
line has been downloaded, it may also need a new configuration;
see the description of the AUTOCONF parameter in Section 3.
INVALID REQUESTS FROM APPLICATIONS. Like other GUARDIAN I/O
processes, CP6100 checks the open id of a requesting process,
and ensures that the device being addressed is up. Each
protocol undertakes some further validation of the request,
to see whether it makes sense in the context of that protocol.
Neither CP6100 nor the protocols keep track of requests
already made; thus, applications are responsible for issuing
requests in the proper order. (For example, a line must be
open before you try to transfer data to or from it.)
October 1985
2-11