Sequence Record (seq)

The Sequence record is used to trigger the processing of up to ten other records and send values to those records. It is similar to the fanout record, except that it will fetch an input value and write an output value instead of simply processing a collection of forward links. It can also specify one of several selection algorithms that determine which values to write. It has no associated device support.

Parameter Fields

The record-specific fields are described below, grouped by functionality.

Scan Parameters

The sequence record has the standard fields for specifying under what circumstances it will be processed. These fields are listed in Scan Fields.

Desired Output Parameters

These fields determine where the record retrieves the values it is to write to other records. All of these values are not necessarily used, depending on the selection algorithm.

The sequence record can retrieve up to 16 values from 16 locations. The user specifies the locations in the Desired Output Link fields (DOL0-DOLF), which can be either constants, database links, or channel access links. If a Desired Output Link is a constant, the corresponding value field for that link is initialized to the constant value. Otherwise, if the Desired Output Link is a database or channel access link, a value is fetched from the link each time the record is processed.

The value fetched from the Desired Output Links are stored in the corresponding Desired Output Value fields (DO0-DOF). These fields can be initialized to a constant value, and may subsequently be changed via dbPuts.

Desired Output Link Fields

Field	Summary	Type	DCT	Default	Read	Write	CA PP
DOL0	Input link 0	INLINK	Yes		Yes	Yes	No
DOL1	Input link1	INLINK	Yes		Yes	Yes	No
DOL2	Input link 2	INLINK	Yes		Yes	Yes	No
DOL3	Input link 3	INLINK	Yes		Yes	Yes	No
DOL4	Input link 4	INLINK	Yes		Yes	Yes	No
DOL5	Input link 5	INLINK	Yes		Yes	Yes	No
DOL6	Input link 6	INLINK	Yes		Yes	Yes	No
DOL7	Input link 7	INLINK	Yes		Yes	Yes	No
DOL8	Input link 8	INLINK	Yes		Yes	Yes	No
DOL9	Input link 9	INLINK	Yes		Yes	Yes	No
DOLA	Input link 10	INLINK	Yes		Yes	Yes	No
DOLB	Input link 11	INLINK	Yes		Yes	Yes	No
DOLC	Input link 12	INLINK	Yes		Yes	Yes	No
DOLD	Input link 13	INLINK	Yes		Yes	Yes	No
DOLE	Input link 14	INLINK	Yes		Yes	Yes	No
DOLF	Input link 15	INLINK	Yes		Yes	Yes	No

Desired Output Value Fields

Field	Summary	Type	DCT	Default	Read	Write	CA PP
DO0	Value 0	DOUBLE	No		Yes	Yes	No
DO1	Value 1	DOUBLE	No		Yes	Yes	No
DO2	Value 2	DOUBLE	No		Yes	Yes	No
DO3	Value 3	DOUBLE	No		Yes	Yes	No
DO4	Value 4	DOUBLE	No		Yes	Yes	No
DO5	Value 5	DOUBLE	No		Yes	Yes	No
DO6	Value 6	DOUBLE	No		Yes	Yes	No
DO7	Value 7	DOUBLE	No		Yes	Yes	No
DO8	Value 8	DOUBLE	No		Yes	Yes	No
DO9	Value 9	DOUBLE	No		Yes	Yes	No
DOA	Value 10	DOUBLE	No		Yes	Yes	No
DOB	Value 11	DOUBLE	No		Yes	Yes	No
DOC	Value 12	DOUBLE	No		Yes	Yes	No
DOD	Value 13	DOUBLE	No		Yes	Yes	No
DOE	Value 14	DOUBLE	No		Yes	Yes	No
DOF	Value 15	DOUBLE	No		Yes	Yes	No

Output Parameters

When the record is processed, the desired output values are retrieved for the links in the record’s selection algorithm and are written to the corresponding output link (LNK0-LNKF). These output links can be database links or channel access links; they cannot be device addresses. There are sixteen output links, one for each desired output link. Only those that are defined are used.

Field	Summary	Type	DCT	Default	Read	Write	CA PP
LNK0	Output Link 0	OUTLINK	Yes		Yes	Yes	No
LNK1	Output Link 1	OUTLINK	Yes		Yes	Yes	No
LNK2	Output Link 2	OUTLINK	Yes		Yes	Yes	No
LNK3	Output Link 3	OUTLINK	Yes		Yes	Yes	No
LNK4	Output Link 4	OUTLINK	Yes		Yes	Yes	No
LNK5	Output Link 5	OUTLINK	Yes		Yes	Yes	No
LNK6	Output Link 6	OUTLINK	Yes		Yes	Yes	No
LNK7	Output Link 7	OUTLINK	Yes		Yes	Yes	No
LNK8	Output Link 8	OUTLINK	Yes		Yes	Yes	No
LNK9	Output Link 9	OUTLINK	Yes		Yes	Yes	No
LNKA	Output Link 10	OUTLINK	Yes		Yes	Yes	No
LNKB	Output Link 11	OUTLINK	Yes		Yes	Yes	No
LNKC	Output Link 12	OUTLINK	Yes		Yes	Yes	No
LNKD	Output Link 13	OUTLINK	Yes		Yes	Yes	No
LNKE	Output Link 14	OUTLINK	Yes		Yes	Yes	No
LNKF	Output Link 15	OUTLINK	Yes		Yes	Yes	No

Selection Algorithm Parameters

When the sequence record is processed, it uses a selection algorithm similar to that of the selection record to decide which links to process.The select mechanism field (SELM) has three algorithms to choose from: All, Specified or Mask.

Record fields related to the Selection Algorithm

Field	Summary	Type	DCT	Default	Read	Write	CA PP
SELM	Select Mechanism	MENU seqSELM	Yes		Yes	Yes	No
SELN	Link Selection	USHORT	No	1	Yes	Yes	No
SELL	Link Selection Loc	INLINK	Yes		Yes	Yes	No
SHFT	Shift for Mask mode	SHORT	Yes	-1	Yes	Yes	No
OFFS	Offset for Specified	SHORT	Yes		Yes	Yes	No

Fields Description

SELM - Selection Mode

Index	Identifier	Choice String
0	seqSELM_All	All
1	seqSELM_Specified	Specified
2	seqSELM_Mask	Mask

See “Selection Algorithms Description” below;

SELL - Link Selection Location

This field can be initialized as a CONSTANT or as a LINK to any other record. SELN will fetch its value from this field when the seq record is processed. Thus, when using Mask or Specified modes, the links that seq will process can be dynamically changed by the record pointed by SELL.

SELN - Link Selection

When SELM has the value Specified the SELN field sets the index number of the link that will be processed, after adding the OFFS field:

LNK_n_ where n = SELN + OFFS

(If not set, the OFFS field is ZERO)

When SELM has the value Mask the SELN field provides the bitmask that determines which links will be processed, after shifting by SHFT bits:

if (SHFT >= 0)
  bits = SELN >> SHFT
else
  bits = SELN << -SHFT

(If not set, the SHFT field is -1 so bits from SELN are shifted left by 1)

Note about SHFT and OFFS fields

The first versions of seq record had DO, DOL, LNK and DLY fields starting with index ONE (DO1, DOL1, LNK1 and DLY1). Since EPICS 7 the seq record now supports 16 links, starting from index ZERO (DO0, DOL0, LNK0 and DLY0). The SHFT and OFFS fields were introduced to keep compatibility of old databases that used seq records with links indexed from one.

To use the DO0, DOL0, LNK0, DLY0 fields when SELM = Mask, the SHFT field must be explicitly set to ZERO

Selection Algorithms Description

All

The All algorithm causes the record to process each input and output link each time the record is processed, in order from 0 to 15. So when SELM is All, the desired output value from DOL0 will fetched and sent to LNK0, then the desired output value from DOL1 will be fetched and sent to the location in LNK1, and so on until the last input and output link DOF and LNKF. (Note that undefined links are not used.) If DOL_x_ is a constant, the current value field is simply used and the desired output link is ignored. The SELN field is not used when All is the algorithm.

Specified

When the Specified algorithm is chosen, each time the record is processed it gets the integer value in the Link Selection (SELN) field and uses that as the index of the link to process. For instance, if SELN is 4, the desired output value from DO4 will be retrieved and sent to LNK4. If DOL_x_ is a constant, DO_x_ is simply used without the value being fetched from the input link.

Mask

When Mask is chosen, the record uses the individual bits of the SELN field to determine the links to process. When bit 0 of SELN is set, the value from DO0 will be written to the location in LNK0; when bit 1 is set, the valud from DO1 will be written to the location in LNK1 etc. Thus for example if SELN is 3, the record will retrieve the values from DO0 and DO1 and write them to the locations in LNK0 and LNK1, respectively. If SELN is 63, DO0…DO5 will be written to LNK0…LNK5.

Delay Parameters

The delay parameters consist of 16 fields, one for each I/O link discussed above. These fields can be configured to cause the record to delay processing the link. For instance, if the user gives the DLY1 field a value of 3.0, each time the record is processed at run-time, the record will delay processing the DOL1, DOV1, and LNK1 fields for three seconds. That is, the desired output value will not be fetched and written to the output link until three seconds have lapsed.

Field	Summary	Type	DCT	Default	Read	Write	CA PP
DLY0	Delay 0	DOUBLE	Yes		Yes	Yes	No
DLY1	Delay 1	DOUBLE	Yes		Yes	Yes	No
DLY2	Delay 2	DOUBLE	Yes		Yes	Yes	No
DLY3	Delay 3	DOUBLE	Yes		Yes	Yes	No
DLY4	Delay 4	DOUBLE	Yes		Yes	Yes	No
DLY5	Delay 5	DOUBLE	Yes		Yes	Yes	No
DLY6	Delay 6	DOUBLE	Yes		Yes	Yes	No
DLY7	Delay 7	DOUBLE	Yes		Yes	Yes	No
DLY8	Delay 8	DOUBLE	Yes		Yes	Yes	No
DLY9	Delay 9	DOUBLE	Yes		Yes	Yes	No
DLYA	Delay 10	DOUBLE	Yes		Yes	Yes	No
DLYB	Delay 11	DOUBLE	Yes		Yes	Yes	No
DLYC	Delay 12	DOUBLE	Yes		Yes	Yes	No
DLYD	Delay 13	DOUBLE	Yes		Yes	Yes	No
DLYE	Delay 14	DOUBLE	Yes		Yes	Yes	No
DLYF	Delay 15	DOUBLE	Yes		Yes	Yes	No

Operator Display Parameters

These parameters are used to present meaningful data to the operator. The Precision field (PREC) determines the decimal precision for the VAL field when it is displayed. It is used when the get_precision record routine is called.

See Fields Common to All Record Types for more on the record name (NAME) and description (DESC) fields.

Field	Summary	Type	DCT	Default	Read	Write	CA PP
PREC	Display Precision	SHORT	Yes		Yes	Yes	No
NAME	Record Name	STRING [61]	No		Yes	No	No
DESC	Descriptor	STRING [41]	Yes		Yes	Yes	No

Alarm Parameters

The sequence record has the alarm parameters common to all record types. Alarm Fields lists the fields related to alarms that are common to all record types.

Record Support

Record Processing

Routine process implements the following algorithm:

1. First, PACT is set to TRUE, and the link selection is fetched. Depending on the selection mechanism chosen, the appropriate set of link groups will be processed. If multiple link groups need to be processed they are done in increasing numerical order, from LNK0 to LNKF.

2. When LNK_x_ is to be processed, the corresponding DLY_x_ value is first used to generate the requested time delay, using the IOC’s Callback subsystem to perform subsequent operations. This means that although PACT remains TRUE, the lockset that the sequence record belongs to will be unlocked for the duration of the delay time (an unlock occurs even when the delay is zero).

3. After DLY_x_ seconds have expired, the value in DO_x_ is saved locally and a new value is read into DO_x_ through the link DOL_x_ (if the link is valid). Next the record’s timestamp is set, and the value in DO_x_ is written through the LNK_x_ output link. If the value of DO_x_ was changed when it was read in a monitor event is triggered on that field.

4. If any link groups remain to be processed, the next group is selected and the operations for that group are executed again from step 2 above.

   If the last link group has been processed, UDF is set to FALSE and the record’s timestamp is set.

5. Monitors are posted on VAL and SELN.

6. The forward link is scanned, PACT is set FALSE, and the process routine returns.

For the delay mechanism to operate properly, the record is normally processed asynchronously. The only time the record will not be processed asynchronously is if it has nothing to do, because no link groups or only empty link groups are selected for processing (groups where both DOL_x_ and LNK_x_ are unset or contain only a constant value).