The sseq (string sequence) record


  1. Introduction
  2. Scan/Control Fields
  3. Desired Output Fields
  4. Output/Wait Fields
  5. Selection Algorithm Fields
  6. Delay Fields
  7. Operator Display Fields
  8. Alarm Fields
  9. Record Support Routines

1. Introduction

The sseq (String Sequence) record is derived from the seq (Sequence) record, and has all of its capabilities. In addition, the sseq record can read from either string or numeric fields, and write to either string or numeric fields; it can wait for processing that any of its output links triggers to complete; and its execution can be aborted. The record can execute a sequence of up to ten sets of delay/get-value/put-value/wait-for-completion operations. All steps and sequences are optional; with default field values, the record does nothing at all. Like the seq record, the sseq record implements several selection algorithms that allow a database programmer, or run-time user, to specify which sets of delay/get/put/wait to execute. The record has no associated device support.

The sseq record's execution can be aborted by the user. When this happens, the record will immediately stop causing other records to process, but it cannot abort any execution it already started with one of its links. Also, aborting the sseq record will not prevent it from executing its forward link.

Beginning with std-module version R3-0, the string field mentioned above can be either a PV of type DBF_STRING, or an array PV whose elements are of type DBF_CHAR or DBF_UCHAR. In any case, the length that will be read or written by the sseq record is limited to the first 39 characters, because the sseq record uses EPICS string PVs internally.

Beginning with std-module version R3-0, the sseq record posts the states of its input and output links (but not its forward link), and posts PVs that indicate which output links are waiting for callbacks.

When writing to a PV of type DBF_MENU or DBF_ENUM, the sseq record always writes its string value. (Examples of such PVs are any record's SCAN field, and the VAL field of a bo or mbbo record.) This is awkward, because it's common practice to write numeric values to such fields, but doing so with the sseq record will not have the expected result. Worse, if there are no strings defined for such a PV, then the sseq record simply cannot write to that PV.

2. Scan/Control Fields

Like all other EPICS records, the sequence record has the standard set of fields (SCAN, PINI, FLNK, PROC, etc.) for specifying when or under what circumstances it should process, and what record, if any, should process immediately afterward. See the EPICS Record Reference Manual for more information.

In addition to standard control fields, the sseq record has the field, ABORT:

Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
ABORT Abort execution SHORT No 0 Yes Yes Yes No

The user, or another EPICS record, can cause a running sseq record to stop executing links by writing '1' to the record's ABORT field. The ABORT field will remain '1' until the record has finished aborting, at which time the field will be reset to '0'. Only the execution of the record itself will stop when it is aborted. Any processing that the record has already started (via its links) will be unaffected by the abort, and the forward link will be executed in any case.

The first write of '1' to the abort field waits for outstanding callbacks to arrive before returning the record to the idle state. This is the preferred way to abort an executing sequence. However, it may be that a callback the record is waiting for will never arrive. In this case, a more thorough abort is required, to put the record back into an executable state. Therefore, if a second write of '1' to the abort field occurs while the record is waiting for callbacks, the record will abandon outstanding callbacks, and return immediately to the idle state. However, any abandoned callbacks remain outstanding, and can arrive at any time. If one comes in while the record is idle, it will be ignored, but if it comes in while the record is executing a fresh sequence, it may be treated as the result of that fresh sequence.

It has become common practice for EPICS developers to treat a sequence record's forward link as an extra LNKn field, and to chain a series of sequence records together using forward links. These practices are not recommended for use with the sseq record, because the forward link is not subject to ABORT; it is always executed.
For previous versions of the record (earlier than std-module version R3-0), users were cautioned not to write to the ABORT the record via a PP link, because this could cause EPICS to reprocess the record after the abort had succeeded. The record now defends itself against this possibility, by clearing its RPRO field as the last step of an abort.

3. Desired Output Fields

These link fields hold the values the record is to write to other records, or determine from where those values are to be read.

The sequence record can fetch up to 10 values from 10 locations. The user specifies the locations in the Desired Output Link fields (DOL1-DOLA), which can be either numeric constants or links to other EPICS PVs. If a Desired Output Link is a numeric constant, the corresponding value field for that link is initialized to the constant value. Otherwise, if the Desired Output Link is a string, it is assumed to represent a link, a value is fetched from the link each time the record is processed. See the EPICS Record Reference Manual for information on how to specify database links. If you want to initialize a string value, use STRn. For example, to write a database file that initializes the string STR1 (the string that will be written by LNK1) to "abcdefg", include the bold line below:

	record(sseq,"mySeqRecord") {

The values fetched from the Desired Output Links are stored in the corresponding Desired Output fields (DO1-DOA and STR1-STRA). These fields can be initialized by a database configuration tool, and they can be changed at run time. But note that the value of DOn and STRn will be overwritten if DOLn is a valid link (i.e., if DOLn contains the name of a PV to which the sseq record is able to make a channel-access connection).

Many EPICS records that have a DOL link field also have an OMSL (Output Mode Select) field, which determines whether or not the DOL field is used. Like the seq record, the sseq record has no such field. Its DOLn fields are treated as links if they contain text that could be a PV name, and value fields are overwritten if those links successfully retrieve values.

Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
DOL1...DOLA Desired Output Link INLINK Yes 0 Yes Yes N/A No
DOL1V...DOLAV Desired Output Link Valid MENU ("Ext PV NC", "Ext PV OK", "Local PV", "Constant") No 0 Yes No Yes No
DO1...D0A Desired Output Value DOUBLE No 0 Yes Yes No No
STR1...STRA Desired Output String STRING No 0 Yes Yes No No

4. Output/Wait Fields

When the sseq record is processed, desired output values (DOn or STRn) are written to the corresponding output links (LNKn). These output links should either be blank (in which case no writing will occur), or contain the name of an EPICS PV; they may not be device addresses. There are ten output links. Only those that contain valid PV names are used. If a link field contains a PV name to which a CA connection cannot be made, the rest of the record currently continues to operate as though the unconnected link field were blank. (This is probably not a good practice, and it is not guaranteed to persist as the record is improved.)

EPICS links can cause processing of the linked-to (i.e., target) record to occur. Whether or not processing actually does occur depends on both the specification of the link, and the properties of the target record. This is well documented in the EPICS Application Developer's Guide, but since some details of link behavior are essential prerequisites for an understanding of the sseq record's WAITn fields, the (EPICS 3.14) behavior of the output links, LNKn, is summarized here:

There are three possibilities:

If LNKn has the attribute PP (e.g., "targetRecord.field PP NMS"), then the link will attempt to cause targetRecord to process. The attempt will succeed, however, only if the target record is "Passive" (e.g., targetRecord.SCAN has the value Passive). In other words, PP means "process if passive".
If LNKn has the attribute CA (e.g., "targetRecord.field CA NMS"), then targetRecord will process only if field has been designated by targetRecord's record-support code as a "Process Passive" field. You can tell if a field is "Process Passive" either by looking at the record's .dbd file, or by writing to it from any Channel Access client; if the record processes when the field in question is written to via Channel Access, then the field is Process Passive, and a CA link to that field from the sseq record will also cause the record to process.
If LNKn has the attribute NPP, then targetRecord will not process as a result of LNKn.

The sseq record is permitted to demand a completion callback from EPICS only if the output-link field it's processing has the attribute CA. A PP link is not permitted to make this demand. If LNKn does have the CA attribute, and WAITn has the value Wait, then the sseq record will demand a completion callback. In this case, it will wait after firing LNKn for the callback, before moving on to the next group of DLY/DOL/LNK fields. If LNKn has any attribute other than 'CA', WAITn is irrelevant and its value will be ignored.

If LNKn has the attribute CA, and WAITn has the value Wait, but targetRecord.field is not a Process-Passive field, then targetRecord will not process as a result of LNKn, but the sseq record will immediately receive a completion callback anyway, and will therefore not wait for targetRecord to process.

Finally, if the sseq record successfully waits for targetRecord to finish processing, it is still possible for other records to process as an indirect result of LNKn, and the sseq record cannot wait for this indirectly caused processing to finish unless the database developer has arranged for the indirectly caused processing to be traceable by EPICS. For example, a channel access client may have a monitor on the field the sseq record writes to, and may do something when that field's value changes. EPICS cannot trace this processing without special help from a database developer. This issue is covered in depth in the documentation of the sscan record, which also relies on EPICS execution tracing to determine when processing it has caused finishes. (See "Completion Reporting" in the Powerpoint presentation "Scans.ppt" in the synApps sscan module's documentation directory.)

Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
LNK1...LNKA Output link OUTLINK Yes blank Yes Yes N/A No
LNK1V...LNKAV Output Link Valid MENU ("Ext PV NC", "Ext PV OK", "Local PV", "Constant") No 0 Yes No Yes No
WAIT1...WAITA Wait Command MENU ("NoWait", "Wait", "After1", ..., "After10") Yes "NoWait" Yes Yes N/A No
These fields determine whether the sseq record waits for completion of processing started by an output link, and at what point in the sequence the record waits. If WAITn has any value other than "NoWait", the record will wait. To accomplish this, the record will attempt to execute LNKn in such a way that the record will be notified when all processing started by the link has finished. This attempt can succeed only if the link has the attribute "CA".

If WAITn has the value "Wait", the record will wait for completion before going on to the next action.

If WAITn has the value "Afteri, where i is in [1,10], then the record will process actions up to and including action i before waiting for completion.

Thus, for example, if you want to execute links 1 and 2 in quick succession, and then wait for completion of processing started by both links, before moving on to link 3, you would set WAIT1 to "After2", and set WAIT2 to either "Wait" or "After2". (For link n, "Wait" means the same thing as "Aftern").

The sseq record cannot go back in time. If you specify "After2" for link 3, the record will behave as though you had specified "Wait" or (equivalently) "After3".

WTG1...WTGA Waiting SHORT No 0 Yes No Yes No
These fields display the waiting states of their respective links. While the record is waiting for completion of processing started by a link, this field will have the value 1.

To avoid any misunderstanding, let me emphasize that WAITn fields do not control or affect the sseq record's use of its delay (DLYn) fields. WAITn only controls whether the record will wait for processing started by LNKn to complete. Thus, if WAITn is 0, the record will execute LNKn and immediately pause for DLYn seconds; if WAITn is 1, the record will execute LNKn, wait for processing started by LNKn to complete, and then pause for DLYn seconds.

By the way, because the sseq record can write strings, it can be used to write to link fields (its own, or those of another record). This use can succeed only if the link field doing the writing has the attribute CA.

5. Selection Algorithm Fields

When the sseq record is processed, it uses a selection algorithm similar to that of the sel (selection) record to decide which links to process. The select mechanism field (SELM) provides three algorithms to choose from: All, Specified or Mask.
All link groups are processed, in order from 1 to 10. Thus, if DOL1 or LNK1 is connected, record will wait for DLY1 seconds, fetch the desired output value from DOL1 (if DOL1 contains a PV name), place it in DO1 and STR1, and send it LNK1 (if LNK1 contains a PV name), optionally waiting for completion. Then the record will move on to process DLY2, DOL2, DO2, STR2, LNK2, WAIT2, and so on until the last input and output link DOA and LNKA. The SELN field is not used when SELM = All.
Each time the record is processed it will get the integer value in the Link Selection (SELN) field and uses that as the index of the link group to process. For instance, if SELN is 4, the desired output value from DO4 will be fetched and sent to LNK4. If DOLn is a constant, DOn is simply used without the value being fetched from the input link.
Each time the record is processed, the record uses the integer value from the SELN field as a bit mask to determine which link groups to process. For example, if SELN is 1, then the value from DO1 will be written to the location in LNK1. If SELN is 3, the record will fetch the values from DO1 and DO2 and write them to the locations in LNK1 and LNK2, respectively. If SELN is 63, DO1...DO6 will be written to LNK1...LNK6.
Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
SELM Select Mechanism RECCHOICE Yes 0 Yes Yes No No
SELN Link Selection USHORT No 1 Yes Yes No No
SELL Link Selection Location INLINK Yes 0 No No N/A No

6. Delay Fields

There are ten delay-related fields, one for each I/O link discussed above. These fields cause the record to delay processing before fetching data from the associated input link, or writing to the associated output link. For example, 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 for three seconds before fetching data from the DOL1 link. If neither an input or an output link exist, the associated delay will be ignored.

Delays are implemented with a time granularity of the system clock, which typically has a frequency of 60 Hz. When a delay value is specified, the record rounds it to the nearest multiple of the system clock period, and writes it back to the DLYn field.
Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
DLY1 Delay time DOUBLE Yes 0 Yes Yes No No
DLY2 Delay time DOUBLE Yes 0 Yes Yes No No
... ... ... ... ... ... ... ... ...
DLYA Delay time DOUBLE Yes 0 Yes Yes No No

7. Operator Display Fields

These fields 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 the EPICS Record Reference Manual for more on the record name (NAME) and description (DESC) fields.
Field Summary Type DCT Initial Access Modify Rec Proc Monitor PP
PREC Display Precision SHORT Yes 0 Yes Yes No No
NAME Record Name STRING [29] Yes 0 Yes No No No
DESC Description STRING [29] Yes Null Yes Yes No No
BUSY Sequence active SHORT No 0 Yes No Yes No

8. Alarm Fields

The sequence record has the alarm fields common to all record types. See the EPICS Record Reference Manual for details.

9. Record Support Routines

The only record support routine is process.

1. First, PACT is set to TRUE, and the link selection is fetched. Depending on the selection mechanism, the link selection output links are processed in order from LNK1 to LNKA. When LNKn is processed, the corresponding DLYn value is used to generate a delay via watchdog timer.
2. After DLYn seconds have expired, the input value is fetched from DOn (if DOLn is constant) or DOLn (if DOLn is a database link or channel access link) and written to LNKn.
3. When all links are completed, an asynchronous completion call back to dbProcess is made (see the EPICS Application Developer's Guide for more information on asynchronous processing.)
4. Then UDF is set to FALSE.
5. Monitors are checked.

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 processed asynchronously. The only time the record will not be processed asynchronously is when there are no non-NULL output links selected (i.e. when it has nothing to do.) The processing of the links is done via callback tasks at the priority set in the PRIO field in dbCommon (see the EPICS Application Developer's Guide for more information.