HP 82479A Owner's Manual page 194

Custom Programs
e group list - (input/output): A string that contains one character for each group in the setup.
Each character is a i' if the group is currently active, or an HM if the group is not active. For
instance, if group $S is active, the fifth character will be ''. The subprogram can modify the
contents of this string to activate or deactivate individual groups.
e memory - (output): A number which indicates the amount of memory required by the
subprogram. This defaults to 500 bytes, enough for many applications, and may be ignored if
500 bytes is sufficient. If the subprogram uses many variables or large arrays, the memory
requirements must be specified, otherwise LI![: may abort with a low memory error. For
instance, if the subprogram dimensions a real array of 100 elements and uses 19 variables, the
memory requirements will be at least 990 bytes. For details on memory requirements of
variables, refer to "System Characteristics" in the HP-71 Reference Manual.
e errorflag - (output): A number which indicates an error resulting from some problem in the
subprogram. This number should be Kl if there are no errors, or 1 if an error is encountered.
If an error is reported, the LI: program will end immediately.
[CAUTION]
There are five things that a custom program should never do:
The custom programs should not alter HP-71 system flags or use flags 0 through 10. These
flags are used for monitoring the status of the event scheduler in the L115 program, as well as
other functions. Flags 11 through 63 are available for use.
The custom programs should not corrupt the instrument buffer. If a second HP 3421A is
used by a custom program, the instrument buffer should be restored.
The custom programs should not use system timers #1 and #2. Timer #3 is available for use.
The custom programs should not alter setup or data files being used by the calling program.
Custom programs for L.I5 should never alter the number of characters in the grouplist, or set
the characters in the string to other than ' or MH.
In the following program example, a device connected to an RS-232 interface is used in a data logging
application. The device returns a reading in response to the command F 1. Note that line 20 is used to tell
the |.I15 program that 5 readings are to be stored. In the event of an error, the device is cleared, and the
error flag 2 is set before returning to L.LI15.