areaDetector: EPICS Area Detector Support

Contents

• Overview
• Architecture
• Implementation details
  • asynPortDriver
  • NDArray
  • NDArrayPool
  • NDAttribute
  • NDAttributeList
  • PVAttribute
  • paramAttribute
  • asynNDArrayDriver
• Detector drivers
  • ADDriver
  • Guidelines and rules for drivers
  • Simulation detector driver
  • Prosilica driver
  • Pilatus driver
  • ADSC driver
  • Roper driver
  • MarCCD driver
  • mar345 driver
  • Firewire Windows driver
  • Firewire Linux driver
  • Perkin-Elmer flat panel driver
• Plugins
  • NDPluginDriver
  • Guidelines and rules for plugins
  • NDPluginStdArrays
  • NDPluginFile
  • NDPluginROI
  • NDPluginColorConvert
• MEDM screens
• Viewers
  • ImageJ Viewer
  • IDL Viewer

Overview

The areaDetector module provides a general-purpose interface for area (2-D) detectors in EPICS. It is intended to be used with a wide variety of detectors and cameras, ranging from high frame rate CCD and CMOS cameras, pixel-array detectors such as the Pilatus, and large format detectors like the MAR-345 online imaging plate.

The goals of this module are:

• Minimize the amount of code that needs to be written to implement a new detector.
• Provide a standard interface defining the functions and parameters that a detector driver must support.
• Provide a set of base EPICS records that will be present for every detector using this module. This allows the use of generic EPICS clients for displaying images and controlling cameras and detectors.
• Allow easy extensibility to take advantage of detector-specific features beyond the standard parameters.
• Have high-performance. Applications can be written to get the detector image data through EPICS, but an interface is also available to receive the detector data at a lower-level for very high performance.
• Provide a mechanism for device-independent real-time data analysis such as regions-of-interest and statistics.
• Provide detector drivers for commonly used detectors in synchrotron applications. These include Prosilica GigE video cameras, IEEE 1394 (Firewire) cameras, ADSC and MAR CCD x-ray detectors, MAR-345 online imaging plate detectors, the Pilatus pixel-array detector, Roper Scientific CCD cameras, and the Perkin-Elmer amorphous silicon detector.

Architecture

The architecture of the areaDetector module is shown below.

From the bottom to the top this architecture consists of the following:

• Layer 1. This is the layer that allows user written code to communicate with the hardware. It is usually provided by the detector vendor. It may consist of a library or DLL, of a socket protocol to a driver, a Microsoft COM interface, etc.
• Layer 2. This is the driver that is written for the areaDetector application to control a particular detector. It is written in C++ and inherits from the ADDriver class. It uses the standard asyn interfaces for control and status information. Each time it receives a new data array it can pass it as an NDArray object to all Layer 3 clients that have registered for callbacks. This is the only code that needs to be written to implement a new detector. Existing drivers range from 800 to 1800 lines of code.
• Layer 3. Code running at this level is called a "plug-in". This code registers with a driver for a callback whenever there is a new data array. The existing plugins implement file saving (NDPluginFile), region-of-interest (ROI) calculations (NDPluginROI), color mode conversion (NDPluginColorConvert), and conversion of detector data to standard EPICS array types for use by Channel Access clients (NDPluginStdArrays). Plugins are normally written in C++ and inherit from NDPluginDriver. Existing plugins range from 300 to 800 lines of code.
• Layer 4. This is standard asyn device support that comes with the EPICS asyn module.
• Layer 5. These are standard EPICS records, and EPICS database (template) files that define records to communicate with drivers at Layer 2 and plugins at Layer 3.
• Layer 6. These are EPICS channel access clients, such as MEDM that communicate with the records at Layer 5. areaDetector includes two client applications that can display images using EPICS waveform and other records communicating with the NDPluginStdArrays plugin at Layer 3. One of these clients is an ImageJ plugin, and the other is a freely runnable IDL application.

The code in Layers 1-3 is essentially independent of EPICS. There are only 2 EPICS dependencies in this code.

1. libCom. libCom from EPICS base provides operating-system independent functions for threads, mutexes, etc.
2. asyn. asyn is a module that provides interthread messaging services, including queueing and callbacks.

In particular it is possible to eliminate layers 4-6 in the architecture shown in Figure 1, providing there is a programs such as the high-performance GUI shown in Layer 3. This means that it is not necessary to run an EPICS IOC or to use EPICS Channel Access when using the drivers and plugins at Layers 2 and 3.

The plugin architecture is very powerful, because new plugins can be written for application-specific purposes. For example, a plugin could be written to analyze images and find the center of the beam, and such a plugin would then work with any detector driver. Plugins are also powerful because they can be reconfigured at run-time. For example the NDPluginStdArrays can switch from getting its array data from a detector driver to an NDPluginROI plugin. That way it will switch from displaying the entire detector to whatever sub-region the ROI driver has selected. Any Channel Access clients connected to the NDPluginStdArrays driver will automatically switch to displaying this subregion. Similarly, the NDPluginFile plugin can be switched at run-time from saving the entire image to saving a selected ROI, just by changing its input source. Plugins can be used to form an image processing pipeline, for example with a detector providing data to a color convert plugin, which feeds an ROI plugin, which feeds a file saving plugin. Each plugin can run in its own thread, and hence in its own core on a modern multi-core CPU.

The use of plugins is optional, and it is only plugins that require the driver to make callbacks with image data. If there are no plugins being used then EPICS can be used simply to control the detector, without accessing the data itself. This is most useful when the vendor provides an API has the ability to save the data to a file and an application to display the images.

What follows is a detailed description of the software, working from the bottom up. Most of the code is object oriented, and written in C++.

Implementation details

The areaDetector module depends heavily on asyn. It is the software that is used for interthread communication, using the standard asyn interfaces (e.g. asynInt32, asynOctet, etc.), and callbacks. In order to minimize the amount of redundant code in drivers, areaDetector has been implemented using C++ classes. The base classes, from which drivers and plugins are derived, take care of many of the details of asyn and other common code.

asynPortDriver

Detector drivers and plugins are asyn port drivers, meaning that they implement one or more of the standard asyn interfaces. They register themselves as interrupt sources, so that they do callbacks to registered asyn clients when values change. They inherit from the asynPortDriver base C++ class that is provided in the asyn module. That base class handles all of the details of registering the port driver, registering the supported interfaces, and registering the required interrupt sources. The asynPortDriver class documentation describes this class in detail.

NDArray

The NDArray (N-Dimensional array) is the class that is used for passing detector data from drivers to plugins. An NDArray is a general purpose class for handling array data. An NDArray object is self-describing, meaning it contains enough information to describe the data itself. It can optionally contain "attributes" (class NDAttribute) which contain meta-data describing how the data was collected, etc.

An NDArray can have up to ND_ARRAY_MAX_DIMS dimensions, currently 10. A fixed maximum number of dimensions is used to significantly simplify the code compared to unlimited number of dimensions. Each dimension of the array is described by an NDDimension structure. The NDArray class documentation describes this class in detail.

NDArrayPool

The NDArrayPool class manages a free list (pool) of NDArray objects. Drivers allocate NDArray objects from the pool, and pass these objects to plugins. Plugins increase the reference count on the object when they place the object on their queue, and decrease the reference count when they are done processing the array. When the reference count reaches 0 again the NDArray object is placed back on the free list. This mechanism minimizes the copying of array data in plugins. The NDArrayPool class documentation describes this class in detail.

NDAttribute

The NDAttribute is a class for linking metadata to an NDArray. An NDattribute has a name, description, data type, value, source type and source information. Attributes are identified by their names, which are case-insensitive. There are methods to set and get the information for an attribute. The NDAttribute class documentation describes this class in detail.

NDAttributeList

The NDAttributeList implements a linked list of NDAttribute objects. NDArray objects contain an NDAttributeList which is how attributes are associated with an NDArray. There are methods to add, delete and search for NDAttribute objects in an NDAttributeList. Each attribute in the list must have a unique name, which is case-insensitive. The NDAttributeList class documentation describes this class in detail.

PVAttribute

The PVAttribute class is derived from NDAttribute. It obtains its value by monitor callbacks from an EPICS PV, and is thus used to associate current the value of any EPICS PV with an NDArray. The PVAttribute class documentation describes this class in detail.

paramAttribute

The paramAttribute class is derived from NDAttribute. It obtains its value from the current value of a driver or plugin parameter. The paramAttribute class is typically used when it is important to have the current value of the parameter and the value of a corresponding PVAttribute might not be current because the EPICS PV has not yet updated. The paramAttribute class documentation describes this class in detail.

asynNDArrayDriver

asynNDArrayDriver inherits from asynPortDriver. It implements the asynGenericPointer functions, for NDArray objects. This is the class from which both plugins and area detector drivers are indirectly derived. The asynNDArrayDriver class documentation describes this class in detail.

The file asynNDArrayDriver.h defines a number of enumerations, including NDStdDriverParam_t, which are the parameters that all NDArray drivers and plugins should implement if possible. These parameters are defined by enum values with an associated asyn interface, and access (read-only or read-write). The EPICS database ADBase.template provides access to these standard driver parameters. The following table lists the standard driver parameters. The columns are defined as follows:

• Enum name: The name of the enum value for this parameter in asynNDArrayDriver.h. There are several EPICS records in ADBase.template that do not have corresponding enum fields, and these are indicated as Not Applicable (N/A).
• asyn interface: The asyn interface used to pass this parameter to the driver.
• Access: Read-write (r/w) or read-only (r/o).
• drvUser string: The string used to look up the parameter in the driver through the drvUser interface. This string is used in the EPICS database file for generic asyn device support to associate a record with a particular parameter. It is also used to associate a paramAttribute with a driver parameter in the XML file that is read by asynNDArrayDriver::readNDAttributesFile
• EPICS record name: The name of the record in ADBase.template. Each record name begins with the two macro parameters $(P) and $(R). In the case of read/write parameters there are normally two records, one for writing the value, and a second, ending in _RBV, that contains the actual value (Read Back Value) of the parameter.
• EPICS record type: The record type of the record. Waveform records are used to hold long strings, with length (NELM) = 256 bytes and EPICS data type (FTVL) = UCHAR. This removes the 40 character restriction string lengths that arise if an EPICS "string" PV is used. MEDM allows one to edit and display such records correctly. EPICS clients will typically need to convert such long strings from a string to an integer or byte array before sending the path name to EPICS. This is easy to do in clients like SPEC, Matlab, and IDL.

Note that for parameters whose values are defined by enum values (e.g NDDataType, NDColorMode, etc.), drivers can use a different set of enum values for these parameters. They can override the enum menu in ADBase.template with driver-specific choices by loading a driver-specific template file that redefines that record field after loading ADBase.template.

epicsSnprintf(
    FullFilename, 
    sizeof(FullFilename), 
    FileFormat, FilePath, 
    Filename, FileNumber);
        

ADDriver

ADDriver inherits from asynNDArrayDriver. This is the class from which area detector drivers are directly derived. The ADDriver class documentation describes this class in detail.

The file ADDriver.h defines a number of enumerations, including ADStdDriverParam_t, which are the parameters that all areaDetector drivers should implement if possible.

Guidelines and rules for drivers

The following are guidelines and rules for writing areaDetector drivers

• Drivers will generally implement one or more of the writeInt32(), writeFloat64() or writeOctet() functions if they need to act immediately on a new value of a parameter. For many parameters it is normally sufficient to simply have them written to the parameter library, and not to handle them in the writeXXX() functions. The parameters are then retrieved from the parameter library with the getIntParam(), getDoubleParam(), or getStringParam() function calls when they are needed.
• If the writeInt32(), writeFloat64() or writeOctet() functions are implemented they must call the base class function for parameters that they do not handle and whose enum value is less than the value of ADLastStdParam, i.e. parameters that belong to a base class.
• Drivers will need to implement the drvUserCreate() function if they have additional parameters beyond those in the asynPortDriver or ADDriver base classes.
• Drivers will generally need to create a new thread in which they run the acquisition task. Some vendor libraries create such a thread themselves, and then the driver must just implement a callback function that runs in that thread (the Prosilica is an example of such a driver).
• The acquisition thread will typically monitor the acquisition process and perform periodic status update callbacks. The details of how to implement this will vary depending on the specifics of the vendor API. There are many existing detector drivers that can be used as examples of how to write a new driver.
• If the detector hardware does not support fixed-period acquisition or muliple-image acquisition sequence (ADNumImages parameter) then these should be emulated in the driver. The simDetector, marCCD and other drivers can be used as examples of how to do this.
• If NDArrayCallbacks is non-zero then drivers should do the following:
  • Call asynNDArrayDriver::getAttributes to attach any attributes defined for this driver to the current array.
  • Call doCallbacksGenericPointer() so that registered clients can get the values of the new arrays. Drivers must release their mutex by calling this->unlock() before they call doCallbacksGenericPointer(), or a deadlock can occur if the plugin makes a call to one of the driver functions.

MEDM screens

The following is the MEDM screen that provides access to the parameters in asynNDArrayDriver.h and ADDriver.h through records in ADBase.template. This is a top-level MEDM screen that will work with any areaDetector driver. Note however that many drivers will not implement all of these parameters, and there will usually be detector-specific parameters not shown in this screen, so detector-specific MEDM screens should generally be created that display the EPICS PVs for the features implemented for that detector.

ADBase.adl

The following is the MEDM screen that provides access to the file-related parameters in ADStdDriverParams through records in NDFile.template. This screen is for use with detector drivers that directly implement file I/O.

NDFile.adl

The following is the MEDM screen that provides access to the EPICS shutter parameters in ADStdDriverParams through records in ADBase.template. This screen allows one to define the EPICS PVs to open the shutter, close the shutter, and determine the shutter status. The values of these PVs for open and close drive and status can also be defined. Note that in many cases the same PV will be used for open and close drive, but in some cases (e.g. APS safety shutters) different PVs are used for open and close.

ADEpicsShutter.adl