EPICS for Dummies

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:

• 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.

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.
• Records vary significantly in complexity (see #More about records)

What is a record field?

• A controllable or informative property 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: used only for troubleshooting to print out all the records that get processed after the record is processed ( setTPRO to 1)
• Note: some of the fields of a record are actually record attributes (they return the same value for all instances of the record type [1])

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:

• It is rare for new records to be created, now that asyn device support exists. add link to asyn?
• areaDetector databases are composed of mostly simple record.

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