The Busy Record

The purpose of the BUSY record is to give EPICS application developers a way to signal the completion of an operation via EPICS' putNotify mechanism (the code that underlies Channel Access' ca_put_callback() function.) putNotify is described in the EPICS Application Developer's Guide (currently, section 5.11); 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 traced 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 traced by EPICS, the callback from EPICS is sufficient to indicate completion, and use of the BUSY record is not warranted. But if you want to include some processing that isn't traced 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 1 must be done by either a ca_put_callback(), or a PP link. (Note that several records in the synApps package of EPICS-application software have output links that are capable of issuing a ca_put_callback(), and that some of them do so via a CA link. Don't be fooled by this. The trick is performed by device support, which calls dbCaPutLinkCallback(), rather than dbPutLink(). dbCaPutLinkCallback() queues a request that eventually results in a ca_put_callback().) The write that resets the record to 0 can be done by a channel-access client, or by device support. If it's done by a CA client, it must be done by either a ca_put() or a CA link.

The BUSY record is essentially a copy of the BO record from EPICS 3.14.8.2, with two modifications: (1)the process routine declines to call recGblFwdLink() when the VAL field has the value 0; (2) the ZNAM and ONAM strings have the default values "Done", and "Busy", respectively.