HP 82479A Owner's Manual page 211

Error Recovery
Some applications may require the system to go to a known-safe state in
case of an equipment or power failure. For example, if a furnace heateris
being controlled, it should go to the off state in case of failure. Note the
following:
The HP 3421A Data Acquisition/Control Unit and the Option 020, 021,
and 022 Multiplexer Assemblies use latching relays. The state of these
relays can only be altered under program control. Under most conditions
of failure, the relays will remain in whatever state the program last set
them,
The HP 3421A Option 050 Digital I/O Assembly output switches are not
latched. Under most conditions of failure the switches will go to the open
state.
It is up to the programmer to provide the appropriate circuitry and
recovery programs to ensure that the system under control goes to a
known-safe state in case of an equipment or power failure. In some
applications serious damage or injury may otherwise occur.
G.5 WRITING YOUR OWN RECOVER PROGRAM
System configurations which must be tolerant of unusual device failure possibilities or multiple loops
(using the HP 82402A Dual HP-IL Adapter) may need specialized error recovery routines that are beyond
the scope of the FEIZOWER program provided in the IIT IL I T% file. The file search order of the HP-71
provides a "hook" that allows a subprogram FELCI%WEERfound in main memory to be executed in place of
the subprogram provided in UTIL IT". (Refer to section 12, "Subprograms and User-Defined Functions"
in the HP-71 Owner's Manual for more about subprograms.)
Several tools are available to the programmer who must build a specialized FEWER subprogram. They
take the form of diagnostic keywords provided in the HP-71, the HP-IL module, and the Data
Acquisition Pac. These keywords can be used to narrow down and repair the error that led to
RECOVER.
The keywords are:
eo ADR3421% - Returns the device specifier of the current HP 3421A.
oe BUF3421% - Returns the contents of the instrument status buffer.
eo ERRL - Returns line number of most recent error or warning.
o ERRM$ - Returns message text of most recent error or warning.
e ERRN - Returns error number of most recent error or warning.
e ON ERROR - Executes a branch when an error occurs.