EPICS for Dummies

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What is EPICS?

EPICS (Experimental Physics and Industrial Control System) is a distributed control system, which means that the IOC (software) can run on a different computer as the user interfaces.


What is an IOC?

IOC (Input/Output Controller) is a piece of software running on a computer:

  • loads databases
  • serves something called process variables (PVs) that you can read and write to
  • used to run on VMEs, but nowadays it runs on anything.
  • usually talks to hardware, but that isn’t a requirement.

IOCs are started with startup scripts. Those scripts tell the IOC what database files to load and where those files are. When the IOC is told to load a database, what it is doing is parsing that database file, identifying the records that are contained within, and then generating the actual epics PVs that a user will interact with.

Records and databases (lists of records) are configuration files describing the initial setup for PVs, the IOC is the software that manages the behavior of PVs and communication to get/set their values.


What is a databases?

Databases are lists of records.


What is a PV?

PV (Process Variable) is a the field from a record: 

PV = record_name + . + field_name

Most of the interactive parts of user displays have PVs associated with them.


What is a record?

Records are the building blocks of EPICS:

  • There is a lot of different types of record, and each type is designed to perform a specific function
  • Records are customizable via fields that allow users to configure how they behave and monitor their state (see #More about record fields to know how to learn about record fields)
  • Records do nothing until they are processed (see #SCAN, PROC & FLNK fields)
  • Records vary significantly in complexity (see #More about records)
  • It is rare for new records to be created, now that asyn device support exists. add link to asyn?


Where do records come from?

Typically, records at the APS comes from either:

  • EPICS base: main core of EPICS
  • synApps: a collection of EPICS modules that are commonly used at the APS

A module is similar to a python package: it is an already written set of databases and associated behavior you can load into your IOC to do certain things, for example talk to specific hardware. An IOC can link to several modules.

Records in synApps modules
Module Records Module Records
alive alive motor motor
asyn asyn optics table
busy busy scaler scaler
calc acalcout, scalcout, sseq, swait, transform sscan sscan
camac camac std epid, timestamp, throttle
lua luascript vac digitel, vs
mca mca vme vme
allenBradley ab1771IFE, ab1771N, ab1771IX, ... allenBradley ..., ab1791, abDcm


If a record isn't listed here, it probably comes from EPICS base. The advantage to using synApps over a collection of modules one assembles on their own is that some amount of BCDA testing has been done.

What is a record field?

Record fields are controllable or informative properties of a record.

Noteworthy fields:

  • VAL - the default field (used if no field is specified in the vast majority of cases)
  • SCAN - determines when a record processes
  • NAME - the record name (helpful if records are aliased)
  • DESC - record description
  • RTYP - the record type
  • DTYP - the device support type (not meaningful for all record types)
  • DISP - disable puts (ca = channel access & pva = pv access) from outside the IOC (when non-zero); e.g. disable a motor from the motorx_all screen
  • DISV/DISA/SDIS - disable record processing internally
    • Record processing is disabled when DISA equals DISV (e.g. uses SDIS field to read in a value/mode, which will disable some control if a certain mode is active)
    • DISA is set to the value of the PV in the SDIS field, if it isn't empty
  • TPRO - trace processing: when set to 1 prints out all the records that get processed after the record is processed; used only for troubleshooting

Note: some of the fields of a record are actually record attributes (they return the same value for all instances of the record type; see https://epics.anl.gov/)


SCAN, PROC & FLNK fields

Records do nothing until they are processed. Records can be processed multiple ways:

  • Manually:
    • Write 1 (or any non-zero value) to a record’s PROC field
    • Forward link (FLNK) from another record
  • Periodically by setting the SCAN field to something other than Passive
SCAN field options
Index Option Index Option
0 Passive 5 2 second
1 Event 6 1 second
2 I/O Intr 7 0.5 second
3 10 second 8 0.2 second
4 5 second 9 0.1 second

Note:

  • If a record’s SCAN field is not specified in a database, it defaults to Passive.
  • The SCAN field is common to all record types and the options can be selected by index instead of by string.
  • I/O Intr processes a record when the driver gets a new value, but is device/driver dependent and isn’t guaranteed to work


Record link types

Much of the power of EPICS comes from the ability to link EPICS records. There are two link implementations: standard and dynamic.

  • Dynamic links (pink) are found on userCalc (swait records) and scan (sscan records) screens; this type of link accept only a PV name. THe role of the process command (see below) is taken in a separate field.
  • Standard links (purple) are found everywhere else.

inlinkHelp and outlinkHelp are accessible from userTransform screens and scaler screens (for the later only).

Input Links

Input links read a value from another PV. It accepts a PV name followed by a process command (e.g. NPP) and an alarm command (e.g. NMS, ignore this). Here are the possible process commands for an input link:

  • NPP - Just read the value; don't process the target record (i.e. read a potentially stale value); this is a default command.
  • PP - Process the target record and then read a value from it.
  • CA - Read a value without causing processing to occur.
  • CP - Process linking record every time the target record processes.
  • CPP - Same as CP, but linking record process only if its SCAN field has the value Passive.

Note: inlinkHelp is accessible from userTransform screens.


Output Links

Output links write a value to another PV. It accepts a PV name followed by a process command (e.g. NPP) and an alarm command (e.g. NMS, ignore this). Here are the possible process commands for an output link:

  • NPP - Just write a value; don't process the target record; this is a default command.
  • PP - Write a value and then process the target record. To be use if you want EPICS to trace processing (= wait for completion)
  • CA - Write a value; let the target record decide whether or not to process (as it would if you typed the value by hand); if the processing does occur, EPICS will not trace it.

Note:

  • The default link process command, NPP, is almost never the desired behavior for output links. Change NPP to PP immediately and revert if problems are discovered.
  • If the target PV is itself a link field, you must specify the command CA; if you want to wait for completion in this case, you must use either the sseq or the sCalcout record, select CA and set the link's WAIT field to Wait
  • outlinkHelp is accessible from userTransform and scaler screens.

What is a device?

More about records

  • Records vary significantly in complexity; one way to classify them would be (*):
    • Simple records are associated with individual data types
    bi, bo, mbbi, mbbo, longin, longout, ai, ao, stringin, stringout, waveform
    • Intermediate records often perform calculations or link multiple records
    calcout, scalcout, acalcout, swait, transform, fanout, dfanout
    • Complex records are associated with devices or higher-level functionality
    motor, scaler, mca, sscan, table, digitel, vs
    • Flexible records allow custom functionality to be implemented
    sub, aSub, luascript

(*) this is not an official classification, nor an exhaustive list of records.

Note that:

  • areaDetector databases are composed of mostly simple record.


More about record fields

To find documentation for a specific field you need to know/find answers to the following questions:

  • Is the field common to all or input/output record types?
  • If not, does the record come from EPICS base or a different EPICS module (synApps)?
    • The EPICS base record reference documentation is the best place to find information about record types that come from EPICS base.
      Field documentation appears in multiple locations:
      • Fields common to all record types
      • Fields common to input record types
      • Fields common to output record types
      • Record-specific documentation
    • synApps record reference documentation


External links

Retrieved from "https://wiki-ext.aps.anl.gov/blc/index.php?title=EPICS_for_Dummies&oldid=166"