Process Variables

A key concept in EPICS, and hence IBEX, is the process variable. In the underlying EPICS framework, a process variable (usually abbreviated to PV) is a named piece of information that can be read/monitored/changed. Information from hardware devices and elsewhere is exposed as process variables, and devices are controlled by writing to the appropriate process variables.

Naming

A process variable name is simply a string of alpha-numeric characters. However, PV names are usually structured to conform to a naming convention, so that individual PV names are unique. The colon “:” character is used to separate the different sections of a PV name. At ISIS we use the following convention:

  • For an instrument all process variables relevant to that instrument will start with the IN: prefix,

  • followed by the instrument name,

  • followed by the device specific details (usually device name and variable name).

For example

  • IN:LARMOR:DAE:GOODUAH is the PV that holds the good microamps on LARMOR

  • IN:IMAT:DAE:GOODUAH is the PV that holds the good microamps on IMAT

  • IN:LARMOR:MK3CHOPPER_01:CH1:FREQ is the PV that holds the frequency of the Mk3 Chopper on LARMOR

Not all PVs belong to an instrument. Items not part of an instrument have a different prefix. For example, the accelerator generates values with a top-level prefix of AC: such as

  • AC:TS1:BEAM:CURR is the PV that holds the TS1 beam current

Displaying

You can read a PV outside of IBEX by starting an EPICS terminal and running:

caget IN:LARMOR:DAE:GOODUAH
camonitor IN:LARMOR:DAE:GOODUAH