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EPICS Record Reference Manual

Chapter 23: Permissive

1. Introduction

The permissive record is for communication between a server and a client. An example would be a sequence program server and an operator interface client. By using multiple permissive records a sequence program can communicate its current state to the client. The fields in this record fall into the following categories:

scan parameters

client-server parameters

operator display parameters

monitor parameters

2. Scan Parameters

The permissive record has the standard fields for specifying under what circumstances the record will be processed. These fields are listed in Scan Fields, Chapter 2, 2. In addition, Scanning Specification, Chapter 1, 1, explains how these fields are used. Since the permissive record supports no direct interfaces to hardware, its SCAN field cannot be I/O Intr.

3. Client-server Parameters

The client and server communicate through the VAL and watchdog flag (WFLG) fields. At initialization, both fields are set equal to 0, which means OFF. The server sets WFLG equal to ON when it is ready to accept a request. The client monitors WFLG and when WFLG equals 1, the client-server action is performed (a private matter between server and client).

When WFLG is off--when the server is busy--the client program may turn the VAL field from OFF to ON. After the server finishes its task, it will notice that VAL is ON and will turn both WFLG and VAL OFF and performs the requested service.

Note that the when WFLG is ON, the client program must not turn VAL to on.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
VAL Value FieldUSHORTNo0YesYesYesYes
WFLG Watchdog FlagUSHORTNo0YesYesYesYes

4. Operator Display Parameters

The label field (LABL) contains a string given to it that should describe the record in further detail. In addition to the DESC field. See Chapter 2, Fields Common to All Record Types, for more on the record name (NAME) and description (DESC) fields.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
LABL LabelSTRING [20]YesNullYesYesNoYes
NAMERecord NameSTRING [29]Yes0YesNoNoNo
DESCDescriptionSTRING [29]YesNullYesYesNoNo

5. Alarm Parameters

The Permissive record has the alarm parameters common to all record types. Alarm Fields, Chapter 2, 3, lists other fields related to a alarms that are common to all record types.

6. Run-time Parameters

These fields are used to trigger monitors for each field. Monitors for the VAL field are triggered when OVAL, the old value field, does not equal VAL. Likewise, OFLG causes monitors to be invoked for WFLG when WFLG does not equal OLFG.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
OFLG Old Flag ValueUSHORTNo0YesNoNoNo

7. Record Support Routines

Two record support routines are provided: process, and get_value.


process sets UDF to FALSE, triggers monitors on VAL and WFLG when they change, and scans the forward link if necessary.


get_value fills in struct valueDes so that it refers to VAL.

1. - Introduction
2. - Scan Parameters
3. - Client-server Parameters
4. - Operator Display Parameters
5. - Alarm Parameters
6. - Run-time Parameters
7. - Record Support Routines

EPICS Record Reference Manual - 19 MAY 1998
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