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EPICS Record Reference Manual

Chapter 38: Wait

Ned D. Arnold

Advanced Photon Source

Argonne National Laboratory

1. Introduction


This chapter describes the capabilities and use of the Wait record. The Wait record was developed primarily to provide "reassignable" link fields that could be changed during run time. A unique library was developed to provide this feature. Since all EPICS standard link fields are "reassignable" in R3.13 and above, the Wait record and its associated library are somewhat obsolete. For new designs, it is recommended that the Calcout record be used, which provides the same function as the Wait record (plus more) while using the standard EPICS links.

The Wait record is derived from the Calc record with the following additional features:

The Wait record also has the capability to process as a result of an input changing (via CA monitors), and it can be used to do conditional processing within the database. Its name is derived from the original requirement that initiated its development-- "I want to wait until all the motors have stopped and then trigger the detector." Its fields fall into the following categories:

scan parameters

read parameters

expression-related parameters

desired output parameters

write parameters

operator parameters

monitor parameters

run-time and simulation mode parameters

2. Scan Parameters


The wait record has the standard fields for specifying under what circumstances the record will be processed. In addition to the standard scan mechanisms available to all records (periodic, passive, event, etc.), the wait record can be specified to process as a result of one of its input values changing (using Channel Access monitors). This offers immediate response to an input change (rather than waiting for the next periodic scan) while minimizing record processing that is not required. The scan mechanism choice I/O Intr will enable this feature. For the current EPICS release, an additional module, caMonitor.o, must be compiled and loaded with the wait record support to provide this feature.
Note: Because of the event driven nature of this feature, it is quite easy to configure a database that results in an infinite loop which uses all available CPU time. If the wait record is set to process as a result of a channel changing and the processing of the wait record causes the channel to change again, an infinite loop will result. The symptom will be a loss of all channel access connections (lower priority tasks) even though the shell responds normally. Using the spy vxWorks utility will confirm the predicament by showing 0% free CPU time.

Scanning Specification, Chapter 1, 1, explains how the rest of these fields are used. They are listed in Scan Fields, Chapter 2, 2.

3. Read Parameters


Like the Calc record, the read parameters for the Wait record consist of 12 input links--INAN, INBN, . . . INLN. The fields can be database links or constants. If they are links, they must specify another record's field in the same IOC. If they are constants, the corresponding value field for the link (A-L) will be initialized with the constant's value and can be changed via dbPuts. The Wait record does not support direct links to hardware.

Unlike the Calc record, these input links can be modified during run time. These are ASCII fields which have a special processing routine attached to them. When changed, the routine is called. The record will use the new link the next time the record is processed.

A consequence of reassignable links is that one cannot force the processing of any specified records prior to retrieving the data from them (i.e. there is no .PP flag). This should be considered when designing a database using the Wait record.

In this initial version, the reassignable links do not support channel access connections external to the IOC. Until this feature is added, the specified Process Variables must reside on the same IOC.

See Address Specification, Chapter 1, 2, for information on specifying links and constants.
FieldSummaryTypeDCTInitialAccessModifyRec Proc Monitor
INANInput Link A NameSTRING[40]Yes0YesYesYes
INBNInput Link B NameSTRING[40]Yes0YesYesYes
INCNInput Link C NameSTRING[40]Yes0YesYesYes
INDNInput Link D NameSTRING[40]Yes0YesYesYes
INENInput Link E NameSTRING[40]Yes0YesYesYes
INFNInput Link F NameSTRING[40]Yes0YesYesYes
INGNInput Link G NameSTRING[40]Yes0YesYesYes
INHNInput Link H NameSTRING[40]Yes0YesYesYes
ININInput Link I NameSTRING[40]Yes0YesYesYes
INJNInput Link J NameSTRING[40]Yes0YesYesYes
INKNInput Link K NameSTRING[40]Yes0YesYesYes
INLNInput Link L NameSTRING[40]Yes0YesYesYes

4. Expression-related Parameters


The expression related fields of the Wait record are identical to that of the Calc record. The two main fields are the CALC and RPCL fields. The CALC field contains the infix expression which the record routine will use when it processes the record. The user enters an expression in this field. The resulting value of the expression is placed in the VAL field The write parameters of the record can write this value if configured to.

The CALC field is actually converted to opcode and stored as a reverse polish expression, or postfix expression, in the RPCL field. It is this expression which is actually used to calculate VAL. CALC can be changed at run-time, and the special record routine calls a function to convert it to the postfix expression. One difference between the Calc record and the Wait record is that the Wait record contains a flag field (CLCV) that indicates if the expression is a valid one. It will be true (non-zero) if the expression is invalid, in which case an error message is generated.

The reverse polish calculation is evaluated most efficiently during run-time. The range of expressions supported by the calculation record are separated into operands, algebraic operations, trigonometric operations, relational operations, logical operations, parentheses and commas, and the question mark operator.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
CALCCalculationDBF_STRINGYes0YesYesYesYes
RPCLReverse PolishDBF_NOACCESSNo0NoNoN/ANo

4.1. Operands

The expression can use the values retrieved from the INPx links as operands, these values being stored in the A-L fields. The values to be used in the expression are simply referenced by the field letter. For instance, the value obtained from the INPA link is stored in field A and the value obtained from INPB is stored in field B. The field names can be included in the expression which will operate on their respective values, as in A+B. In addition, the RNDM unary function can be included as an operand to generate a random number between 0 and 1.
FieldSummaryTypeDCTInitialAccessModifyRec Proc Monitor
AInput Values ADOUBLENo0YesYes/NoYes
BInput Values BDOUBLENo0YesYes/NoYes
CInput Values CDOUBLENo0YesYes/NoYes
DInput Values DDOUBLENo0YesYes/NoYes
EInput Values EDOUBLENo0YesYes/NoYes
FInput Values EDOUBLENo0YesYes/NoYes
GInput Values GDOUBLENo0YesYes/NoYes
HInput Values HDOUBLENo0YesYes/NoYes
IInput Values IDOUBLENo0YesYes/NoYes
JInput Values JDOUBLENo0YesYes/NoYes
KInput Values KDOUBLENo0YesYes/NoYes
LInput Values LDOUBLENo0YesYes/NoYes

4.2. Algebraic Operators

ABS: Absolute value (unary)

SQR: Square root (unary)

MIN: Minimum (binary function)

MAX: Maximum (binary function)

CEIL: Ceiling (unary)

FLOOR: Floor (unary)

LOG: Log base 10 (unary)

LOGE: Natural log (unary)

EXP: Exponential function (unary)

^ : Exponential (binary)

** : Exponential (binary)

+ : Addition (binary)

- : Subtraction (binary)

* : Multiplication (binary)

/ : Division (binary)

% : Modulo (binary)

NOT: Negate (unary)

4.3. Trigonometric Operators

SIN: Sine

SINH: Hyperbolic sine

ASIN: Arc sine

COS: Cosine

COSH: Hyperbolic cosine

ACOS: Arc cosine

TAN: Tangent

TANH: Hyperbolic tangent

ATAN: Arc tangent

4.4. Relational Operators

>= : Greater than or equal to

> : Greater than

<= : Less than or equal to

<: Less than

# : Not equal to

= : Equal to

4.5. Logical Operators

&& : And

|| : Or

! : Not

4.6. Bitwise Operators

| : Bitwise Or

& : Bitwise And

OR : Bitwise Or

AND: Bitwise And

XOR: Bitwise Exclusive Or

~ : One's Complement

<< : Left shift

>> : Right shift

4.7. parentheses and Comma

The open and close parentheses are supported. Nested parentheses are supported.

The comma is supported when used to separate the arguments of a binary function.

4.8. Conditional Expression

The "C" question mark operator is supported. The format is:

  (condition)? True result : False result

4.9. Examples

Algebraic

A + B

Relational

(A + B) < (C + D)

Question Mark

(A+B)<(C+D)?E:F
(A+B)<(C+D)?E

Logical

A&B

5. Desired Output Parameters


Using the choices in the Data Output field (DOPT) explained in the next section, the Wait record can be configured to write as its output the result of its calculation (the value in VAL) or it can be configured to write a desired output value, fetched from an input link. The desired output fields for the Wait record are similar to the desired output parameters of other output records.

In the desired output location (DOLN) field, the user specifies a link from which the record will retrieve the value that it writes (if the DOPT field specifies DOLD). The value is retrieved and placed in the desired output location data field (DOLD). If DOLN specifies no link, a flag field indicates that the DOLN field is invalid. When the DOLN field is invalid, the value of DOLD can be set via dbPuts at run-time.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
DOLN Desired Output LocationSTRING [40]YesNullYesYesNoNo
DOLDDesired Output Location Data DOUBLEYes0YesYesYesNo

6. Write Parameters


The Wait record can write either the result of its calculation (VAL) or the value retrieved by the DOLN field. If the user specifies Use VAL in the Data Option (DOPT) field, then the value in VAL will be written. If the user specifies Use DOL, the value retrieved from the DOLN link and contained in the DOLD field will be written. The DOPT field can be changed during run-time.

The Output Link Name (OUTN) specifies where the record is to write the output value.

The Wait record does not have to write its output every time the record processes. This allows "downstream" processing of records to be done conditionally. The record has output execution options which can be specified in the OOPT field. These options are as follows.

The Wait record can also post an event. If a non-zero value is entered into the OEVT (Output Event) field, the record will "post an event" (using the entered number as the event number) whenever the output link is executed. This is a way of initiating several other records to process as a result of a calculation.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
OOPTOutput OptionRECCHOICEYes0YesYesNoNo
OUTN Output Link Name STRING [40]YesNullYesYesNoNo
DOPTData OptionRECCHOICEYes0YesYesNoNo
OEVT Output Event USHORTYes0YesYesNoNo

7. Operator Display Parameters


These parameters are used to present meaningful data to the operator. They display the value and other parameters of the Wait record either textually or graphically.

The HOPR and LOPR fields set the upper and lower display limits for the VAL field. The get_graphic_double retrieves these fields.

The PREC field determines the floating point precision with which to display VAL. It is used whenever the get_precision record support routine is called.

See Chapter 2, Fields Common to All Record Types, for more on the record name (NAME) and description (DESC) fields.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
HOPRHigh Operating RangeFLOATYes0YesYesNoNo
LOPRLow Operating Range FLOATYes0YesYesNoNo
PRECDisplay PrecisionSHORTYes0YesYesNoNo
NAMERecord NameSTRING [29]Yes0YesNoNoNo
DESCDescriptionSTRING [29]YesNullYesYesNoNo

8. Alarm Parameters


The wait 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.

9. Monitor Parameters


These parameters are delta values that implement a deadband on the value-change monitors for the VAL field. Monitors are sent when the value field exceeds the last monitored field (MLST, for instance) by the appropriate delta. If these fields have a value of zero, everytime the value changes, a monitor will be triggered; if they have a value of -1, everytime the record is processed, monitors are triggered. The ADEL field is used for archive monitors and the MDEL field for all other types of monitors. See Monitor Specification, Chapter 1, 5, for a complete explanation of monitors.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
ADELArchive DeadbandDOUBLEYes0YesYesNoNo
MDELMonitor, i.e. value change, DeadbandDOUBLEYes0YesYesNoNo

10. Run-time Parameters


These parameters are used by the run-time code for processing the Wait record. They are not configurable. They represent the current state of the record. The record support routines use some of them to process the record or to implement monitors.

The OVAL field is used to implement monitors on the VAL field. If VAL differs from OVAL (taking into account the deadband) then monitors for VAL are triggered.

The LA-LL fields are used to implement monitors for the A-L field, the fields which hold the values retrieved from the input links. They hold the values retrieved from INPA-INPL the last time the record was processed. For instance, when A, the current value retrieved from INPA, does not equal LA, the last value retrieved, then monitors for A are triggered.

The DOLV and CLCV are flag fields. The DOLV flag is used to indicate if the process variable specified in the Desired Output Location (DOLN) field is a valid name. If it is not, the desired output value is not retrieved from DOLN. The CLCV flag is used to indicate if the Wait record's CALC expression is a mathematically valid one.

The CBST field contains a pointer to a record private structure. It is not of concern to the user.
FieldSummaryTypeDCTInitialAccessModifyRec Proc MonitorPP
CBSTCallback StructureNOACCESSNoNullNoNoNoNo
OVALOld Value DOUBLENo0YesYesNoNo
DOLA Desired Output Location AddressNOACCESSNoNullNoNoNoNo
DOLVDesired Output Location ValidLONGNo0YesYesYesNo
ALSTArchive Last ValueDOUBLENo0YesNoNoNo
MLSTMonitor Last Value DOUBLENo0YesNoNoNo
CLCV Calculation String ValidLONGNo0YesYesYesNo
LAPrevious InputDOUBLENo0YesYesNoNo
LBPrevious InputDOUBLENo0YesYesNoNo
LCPrevious InputDOUBLENo0YesYesNoNo
LDPrevious InputDOUBLENo0YesYesNoNo
LEPrevious InputDOUBLENo0YesYesNoNo
LFPrevious InputDOUBLENo0YesYesNoNo
LGPrevious InputDOUBLENo0YesYesNoNo
LHPrevious InputDOUBLENo0YesYesNoNo
LIPrevious InputDOUBLENo0YesYesNoNo
LJPrevious InputDOUBLENo0YesYesNoNo
LKPrevious InputDOUBLENo0YesYesNoNo
LLPrevious InputDOUBLENo0YesYesNoNo

1. - Introduction
2. - Scan Parameters
3. - Read Parameters
4. - Expression-related Parameters
4.1. - Operands
4.2. - Algebraic Operators
4.3. - Trigonometric Operators
4.4. - Relational Operators
4.5. - Logical Operators
4.6. - Bitwise Operators
4.7. - parentheses and Comma
4.8. - Conditional Expression
4.9. - Examples
Algebraic
Relational
Question Mark
Logical
5. - Desired Output Parameters
6. - Write Parameters
7. - Operator Display Parameters
8. - Alarm Parameters
9. - Monitor Parameters
10. - Run-time Parameters

EPICS Record Reference Manual - 19 MAY 1998
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