The Busy Record

The purpose of the BUSY record is to give the database developer a way to signal the completion of an operation via EPICS' putNotify/ca_put_callback mechanism. This mechanism is described in the EPICS Application Developer's Guide; for purposes here, it is essentially an execution trace. When ca_put_callback() causes a record to process (i.e., to execute) EPICS takes note of that processing, and any other record processing that results directly from it. "Directly" means that the processing is caused by either a forward link, or a database link with the attribute 'PP'. Only this kind of processing is traceable by EPICS. When all traced processing completes, EPICS sends a callback to the source of the original ca_put_callback() command.

If all of the processing that results from a ca_put_callback() is traceable by EPICS, the callback from EPICS is all you need to indicate completion. But if you want to include some processing that isn't traceable by EPICS, the BUSY record is a convenient way to fold it in. If you arrange for a BUSY record to be processed as a traceable result of the ca_put_callback(), and cause its VAL field to have the value "Busy" (1), the BUSY record will pretend to be doing something (i.e., it will not execute its forward link) as long as its VAL field remains "Busy". When VAL is set to "Done", and the record is processed, it will execute its forward link, notifying EPICS that it is finished, and EPICS will send the callback. The write that sets a BUSY record to "Busy" (1) must be done by either a ca_put_callback(), or a PP link. The write that resets the record to "Done" (0) must be done by either a ca_put() or a CA link.

The busy record has the standard fields (dbCommon.db) for specifying under what circumstances the record will be processed, etc. These fields are listed in the EPICS Record Reference Manual. The BUSY record has four additional fields:

The record's value, which can be either "Busy" (1), or "Done" (0). While VAL is "Busy", the record will not call recGblFwdLink(). If VAL is "Done", the record will call recGblFwdLink() as the last step in processing. When VAL is written to by a channel-access put, the record will process.
The previous value of VAL. (private)
OutputModeSeLect -- This field can take one of two values: "supervisory", which means that VAL accepts whatever value is given to it; and "closed_loop", which means that the value retrieved from the PV named by the DOL field is placed into VAL whenever the record processes, and anything that might have been written to VAL is ignored.
DesiredOutputLocation -- names a PV whose value will be placed into VAL when the record processes. DOL may be specified as a number, instead of a PV name. In this case, VAL is initialized to that number at boot time, and DOL has no further use.