NB: This manual documents a 20+ year old
version of EPICS, see
here for the EPICS 7 Record Reference documentation.
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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.
| Field | Summary | Type | DCT | Initial | Access | Modify | Rec Proc Monitor | PP |
| VAL | Value Field | USHORT | No | 0 | Yes | Yes | Yes | Yes |
| WFLG | Watchdog Flag | USHORT | No | 0 | Yes | Yes | Yes | Yes |
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.
| Field | Summary | Type | DCT | Initial | Access | Modify | Rec Proc Monitor | PP |
| LABL | Label | STRING [20] | Yes | Null | Yes | Yes | No | Yes |
| NAME | Record Name | STRING [29] | Yes | 0 | Yes | No | No | No |
| DESC | Description | STRING [29] | Yes | Null | Yes | Yes | No | No |
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.
| Field | Summary | Type | DCT | Initial | Access | Modify | Rec Proc Monitor | PP |
| OVAL | Old Value | USHORT | No | 0 | Yes | No | No | No |
| OFLG | Old Flag Value | USHORT | No | 0 | Yes | No | No | No |
7. Record Support Routines
Two record support routines are provided: process, and get_value.
process
process sets UDF to FALSE, triggers monitors on VAL and WFLG when they change, and scans the forward link if necessary.
get_value
get_value fills in struct valueDes so that it refers to VAL.
- 1. - Introduction
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- 2. - Scan Parameters
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- 3. - Client-server Parameters
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- 4. - Operator Display Parameters
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- 5. - Alarm Parameters
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- 6. - Run-time Parameters
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- 7. - Record Support Routines
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- process
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- get_value
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EPICS Record Reference Manual - 19 MAY 1998
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