PV Access Support for EPICS Qt

Normative Types
Build Considerations
New/Modified Widgets
Protocol Selection


This page provide information regarding the introduction of the use of the PV Access protocol into EPICS Qt.

The PV Access protocol has been available as EPICS 4 modules for a number of years and has recently been merged into EPICS base as part of EPICS 7. As the Australian Synchrotron is planning to include some EPICS 7 functionality, especially on new beam-lines, we have decided to add support for PV Access to the EPICS Qt (QE) framework.

This support is being introduced as part of the EPICS Qt 3.7 release.

Normative Types

For the 3.7.1 release, the following normative types are be supported.

Support for other image types will be part of a later 3.7 release. Support for other normative types will be added as and when needed.

Build Considerations

Building EPICS

If EPICS Qt is to be built with PV Access support - see below - then you will need EPICS 7 or later. While I suspect it would be possible to add PV Access functionality by building against EPICS 3.15.5 and EPICS 4.6, the inclusion of PV Access into the EPICS Qt framework is assumes you are using EPICS 7.

There are two things to be aware of.


The PV Access modules make extensive use of C++ templates, and I ran into some name mangling issues when building the QE framework against base- Adding the following line to base/configure/os/CONFIG_SITE.Common.linux-x86_64

OPT_CXXFLAGS += -std=gnu++11

fixed this problem. Thanks to Michael Davidsaver for suggesting the solution. This is one of the flags that Qt uses when compiling the QE framework. It was easier to build EPICS base with this flag that rebuild Qt without it.



For base-7.0.2, I found an apparent bug in nttable.cpp which needs fixed to QENTTablethe use of the QENTTable widget.

At line 274 in the NTTable::getColumnNames function, replace

return pvNTTable->getStructure()->getFieldNames();


return pvValue->getStructure()->getFieldNames();

This has since been fixed in base-7.0.3.

Building ACAI

While not directly PV Access related, a significant change to the EPICS Qt framework for release 3.7.1 is that the low level Channel Access code has been dropped, and has been replaced by use of the ACAI library. This is available from:


The need to include ACAI is irrespective of whether PV Access functionality is or is not included. See the getting started page for more information.

Building EPICS Qt 3.7

While I suspect it would be possible to add PV Access by building against EPICS 3.15.5 and EPICS 4.6, the inclusion of PV Access into the EPICS Qt framework is selected by checking for EPICS 7 or later.

When performing a headless build, the qeframeworkSup/Makefile checks the EPICS version and sets the QE_PVACCESS_SUPPORT environment variable accordingly:

ifdef BASE_7_0

The framework.pro project file checks for this environment variable and if set to ‘YES’, then adds QE_PVACCESS_SUPPORT to the set of DEFINES. It also adds pvData, pvAccess and nt to the set of required libraries.

The inclusion of the PV Access functionality depends using EPICS base 7. If not using EPICS base 7, the qmake phase outputs a warning message, but build process continues without the PV Access functionality.

If building the EPICS Qt framework using qtcreator, or explicitly calling qmake and make then you will have to explicitly set the QE_PVACCESS_SUPPORT environment variable.

The EPICS Qt framework code also has a checks the EPICS base version if the QE_PVACCESS_SUPPORT macro defined (see QEPvaCheck.h), and will:

#error Including PV ACCESS support requires EPICS base 7 or later.

if the QE_PVACCESS_SUPPORT macro is defined and EPICS_VERSION < 7. If you intend to try building PVA support using base 3.15 and EPICS 4.6 then you will have to modify the QEPvaCheck.h file.

New/Modified Widgets

A new widget QENTTable has been created to specifically support the NTTableData normative type.

The QEImage widget has been updated to support the NTNDArrayData normative type. Currently this only supports mono 8-bit pixel images.

All EPICS aware widgets may read/display and/or write using PV Access if the underlying normative type is suitable. For example a QELabel can handle PV Access data using NTScalar or NTEnum types, but cannot display NTTable or NTNDArray data.

Protocol Selection

All widgets’ variable (PV) properties may now be prefixed by <protocol>:// in order to specify the protocol to be used. The allowed protocols are ca:// for Channel Access, and when PV Access support included, pva:// for PV Access.

The default protocol when no protocol has been specified is Channel Access (ca://). In future releases, the default protocol might become selectable by use of an environment variable.

The functionality that determines the required protocol strips off the characters before the first :// in the specified variable name and uses this to select the protocol. This has the following consequence: if using a PV whose name that actually contains the character sequence “://”, then you must explicitly proceeded the PV name with a protocol specifier.

In this respect, 3.7 is not quite backward compatible with 3.6; even if the PV Access functionality is not included. In practice, this should not be a problem unless you have a weird PV naming convention.


ca://CURRENT_MONITOR   - connect to PV 'CURRENT_MONITOR' using Channel Access.
pva://CURRENT_MONITOR  - connect to PV 'CURRENT_MONITOR' using PV Access.
CURRENT_MONITOR        - connect using default protocol, Channel Access.
xyz://FRED             - unknown protocol - this results in a message on stderr:
                         PV protocol identification failed for: "xyz://FRED"
ca://WEIRD//:NAME      - connect to PV 'WEIRD//:NAME' using Channel Access.
Last updated: Thu Sep 26 16:46:20 AEST 2019