PVA Links

Since 1.3.0

JSON Reference

A PVA link JSON schema definition file is available.

Default values for all keys.

record(longin, "tgt") {}
record(longin, "src") {
    field(INP, {pva:{
        pv:"tgt",
        field:"",
        local:false,
        Q:4,
        pipeline:false,
        proc:none,
        sevr:false,
        time:false,
        monorder:0,
        retry:false,
        always:false,
        defer:false
    }})
}

Certain keys are only relevant with certain link types.

Key	Input	Output	Forward
pv	X	X	X
field	X	X	X
local	X	X	X
proc		X	X
retry		X	X
atomic	X
defer	X
monorder	X
pipeline	X
Q	X
sevr	X
time	X
always

pv names the target PV in full.

field selects a sub-structure of the target PV. By default "" the top level structure is used. If the target field is a Structure, then .value and various NTScalar meta-data fields are used. Otherwise, the the targeted field is used as the value without any meta-data.

local set to require that pv reference a record in the local database.

Q requests a certain subscription queue depth.

pipeline selects per-subscription flow control.

proc may be set to:

• null (default). Server selects "NPP" or "PP" behavior.

• true / "PP"". Requests target recorded be “processed” after value update.

• false / "NPP". Requests only value update.

• "CP" Subscribes to target PV. Process this record on each update as well as (dis)connect event.

• "CPP". Equivalent to "CP" if SCAN=Passive. Otherwise behaves like "NPP".

sevr controls if and how alarm state of target PV is applied to Maximize Severity.

• "NMS" (default) Target PV severity is not considered.

• "MS" All target PV severities are considered.

• "MSI" Only INVALID severity is considered.

• "MSS" Currently equivalent to "MSI".

time If true (not default) and the selected field of the target PV has a timeStamp sub-structure, then this time will be written to the record TIME field. In almost all cases if will also be necessary to set field(TSE, "-2").

retry. For an output link, and true (not default), then the most recent incomplete PUT will be re-tried when the target PV (re)connects.

always. Ignored. false (default).

atomic. When true (not default) then "CP" and "CPP" link processing will be done all related records locked (cf. dbLockMany()). If several records with atomic:true are linked to different structure fields of the same target this PV, then all records will be locked together and all resulting processing will be atomic.

monorder the relative ordering (increasing) of processing of "CP" and "CPP" records linked to the same target PV.

defer. If true (not default) this output link will only cache the value to be PUT, but will not issue a network operation. Can be used to update several structure fields in a single network PUT.

Input Links

Creating a pva link will result in a subscription being made to the target PV. Updates will be accumulated in a local cache. Link processing reads the most recent value/alarm/time from this cache. While disconnected, a link will read the most recent value/time, with an INVALID alarm severity.

record(longout, "src") {
    field(INP, {pva:{pv:"target:pv"}})
}

To trigger record processing when a monitor update arrives, add the proc:"CP" modifier.

record(longout, "src") {
    field(INP, {pva:{
        pv:"target:pv",
        proc:"CP"
    }})
}

Output Links

The default behavior of a pva output link is to write the link value into a local cache, and then immediately flush that cache and into a network PUT operation. defer:true allows prevents a link from immediately flushing, which allows changes to multiple fields to be combined into a single PUT (eg. writing to QSRV Groups).

By default, an output link will write the .value structure field of the target PV. Any processing is at the discretion of the server. (hint, look for pp(TRUE) or pp(FALSE) in *Record.dbd)

record(longout, "src") {
    field(OUT, {pva:"target:pv"})
}

or equivalently:

record(longout, "src") {
    field(OUT, {pva:{
        "pv":"target:pv"
    }})
}

Additionally, as with DB links, a link may request control of target processing by setting the proc: key with true / "PP"", or the opposite with false / "NPP".

Forward Links

A pva forward link will send an empty PUT request (no field changes) to the target PV with proc:true. If the target PV is a record, then this is equivalent to a PUT of .PROC.