areaDetector Pixirad driver

author:Mark Rivers, University of Chicago


This is an areaDetector driver for the Pixirad detectors from Pixirad.

The Pixirad detector is a pixel array detector with a cadmium teluride (CdTe) sensor, so it has high efficiency at high energy. There are 2 versions of the ASIC, the older PII, and the newer PIII. The PII base module is 476 x 512 pixels, while the PIII is 402 x 512 pixels. The detector is available with a single module (Pixirad-1), 2 modules (Pixirad-2), and 8 modules (Pixirad-8).

The detector does photon counting of all photons above a programmable energy threshold. The detector hardware has 2 energy thresholds and two counters per pixel, so it can collect 2 energy images simultaneously. The PII firmware supports 4 thresholds, by first collecting 2 energies, and then collecting another image with 2 additional energies. The detector can be operated in 1, 2, or 4 color mode. In addition it supports deadtime-free (DTF) counting modes where one image is being collected while the other is being read out. Both 1 and 2 color DTF modes are supported. The NDArrays produced by this driver are unsigned 16-bit integers. They have dimensions [XSIZE, YSIZE] for 1 color images, [XSIZE, YSIZE, 2] for 2 color images, and [XSIZE, YSIZE, 4] for 4 color images. XSIZE=476 for PII and 402 for PIII. YSIZE=NumModules*512 for both the PII and PIII.

The command interface to the detector is via a TCP/IP socket interface to the detector on port 2222.

The driver receives status information from the detector via UDP broadcast messages on port 2224. The status includes temperatures, humidity and high voltage.

The driver receives image data from the detector via UDP broadcast messages on port 2223 for the Pixirad-1, and port 9999 for the Pixirad-2 and Pixirad-8. This data is received by a “UDP listener thread” in the driver, which passes the UDP buffers as they are received via a pointer on an epicsMessageQueue to a “data unpacking thread”. This second thread unscrambles the UDP buffers, converts them to areaDetector NDArrays, and does the callbacks to registered plugins. The size of the epicsMessageQueue is controlled by a parameter passed to the driver constructor, and is set to 1500 in the example IOC. This provides a buffer in case the UDP listener thread is receiving images faster than the data unpacking thread can process them.

The detector can in principle be on a public network where it will receive its IP address from a DHCP server. In practice the current detector firmware does not work correctly in this configuration, because the UDP messages do not use the correct network addresses. This means that the detector must be placed on a private network with no DHCP server. It will always have the network address This also means that the computer running the areaDetector driver must have 2 network cards, with the one connected to the detector having a host address on the 192.168.0 subnet.

This driver inherits from ADDriver. It implements many of the arameters in asynNDArrayDriver.h and in ADArrayDriver.h. It also implements a number of parameters that are specific to the Pixirad detectors. The Pixirad class documentation describes this class in detail.

Implementation of standard driver parameters

The following table describes how the Pixirad driver implements some of the standard driver parameters.

Implementation of Parameters in asynNDArrayDriver.h and ADDriver.h, and EPICS Record Definitions in ADBase.template and NDFile.template
Parameter index variable EPICS record name Description
ADFrameType $(P)$(R)FrameType

The choices for the Pixirad are:

“1 color low”
One color image using threshold 1. On the PII this image is all photons with energies above threshold 1. On the PIII this is all photons with energies above threshold 1 and below threshold 2.
“1 color high”
One color image using threshold 2. In principle threshold 1 is ignorred in this mode. However, on the PIII this only works correctly if threshold 1 is less than threshold 2.
“2 color”
Two color image using thesholds 1 and 2.
“4 color”
Four color image using thesholds 1, 2, 3, and 4.
“1 color DTF”
One color deadtime-free image using theshold 1.
“2 color DTF”
Two color deadtime-free image using theshold 1 for color 1, and threshold 3 for color 2. Note that the dimensions of the NDArrays from the Pixirad driver depend on the value of FrameType. 1 color arrays are 2-D with dimensions [SIZEX, SIZEY]. 2 color arrays are 3-D with dimensions [SIZEX, SIZEY, 2]. 4 color arrays are 3-D with dimensions [SIZEX, SIZEY, 4].
ADTriggerMode $(P)$(R)TriggerMode

The choices for the Pixirad are:

The timing is internal to the detector.
Each external trigger pulse starts the next image. The exposure time is controlled internally by the AcquireTime record.
The rising edge of the external trigger signal starts the next image. The detector continues to acquire while the external signal is high, and then reads out the detector on the falling edge of the external trigger signal.

Note that the minimum time between external trigger pulses is determined by the FrameType record. This is discussed in the Performance measurements section below.

ADTemperature $(P)$(R)Temperature The setpoint for the cold temperature of the detector.
ADTemperatureActual $(P)$(R)TemperatureActual The readback of the temperature on the cold side of the Peltier cooler.
ADNumImages $(P)$(R)NumImages Controls the number of images to acquire.
ADAcquirePeriod $(P)$(R)AcquirePeriod Controls the period between images. If this is greater than the acquisition time then the detector will wait until the period has elapsed before collection the next image.

Pixirad specific parameters

The Pixirad driver implements the following parameters in addition to those in asynNDArrayDriver.h and ADDriver.h. Note that to reduce the width of this table the parameter index variable names have been split into 2 lines, but these are just a single name, for example AutoCalibrate.

Parameter Definitions in pixirad.cpp and EPICS Record Definitions in pixirad.template
Description drvInfo string EPICS record name EPICS record type
System information
A string containing information about the detector, read directly from the detector. SYSTEM_INFO $(P)$(R)SystemInfo waveform
Counting mode (PIII only)

Selects the counting mode. Choices are:

  • 0: Normal
  • 1: NPI This stands for Neighbor Pixel Inhibit. It prevents counting a photon more than once when charge-sharing occurs., 2: NPISUM This stands for Neighbor Pixel Inhibit with Summation. It sums the charge from adjacent pixels when charge-sharing occurs.,
COUNT_MODE $(P)$(R)CountMode, $(P)$(R)CountMode_RBV mbbo, mbbi
Cooling parameters
The state of the Peltier cooler. Choices are “Off” (0) and “On” (1). COOLING_STATE $(P)$(R)CoolingState, $(P)$(R)Cooling_RBV bo, bi
The readback of the temperature (C) on the hot side of the Peltier cooler. HOT_TEMPERATURE $(P)$(R)HotTemperature_RBV ai
The readback of the ambient temperature (C) in the detector box. BOX_TEMPERATURE $(P)$(R)BoxTemperature_RBV ai
The readback of the ambient relative humidity (%) in the detector box. BOX_HUMIDITY $(P)$(R)BoxHumidity_RBV ai
The calculated dew point (C) based on the BoxHumidity. DEW_POINT $(P)$(R)DewPoint_RBV ai
The power level of the Peltier cooler (%). PELTIER_POWER $(P)$(R)PeltierPower_RBV ai

The status of the cooling system. Values are:

0 - “OK”
Cooling status OK.
1 - “Dew Pt Warning”
This means that the cold temperature is within 3 degree of the dew point.
2 - “Dew Pt Error”
This means that the cold temperature is less than or equal to the dew point.
3 - “T Hot Warning”
This means that the hot temperature is greater than 40 C.
4 - “T Hot Error”
This means that the hot temperature is greater than 50 C.
5 - “T Cold Warning”
This means that the cold temperature is greater than 30 C.
6 - “T Cold Error”
This means that the cold temperature is greater than 40 C.

If the CoolingStatus_RBV is any of the Error states then the driver will automatically turn off the Peltier cooler.

COOLING_STATUS $(P)$(R)CoolingStatus_RBV mbbi
High voltage parameters

High voltage mode. Choices are:

  • 0 - “Manual”
  • 1 - “Auto”

In Manual mode the high voltage is turned off and on with the HVState record. In Auto mode if HVState is Off then the high voltage will be automatically turned on when an acquisition is started and automatically turn off when the acquisition is complete. This can improve the image quality, because the detector is subject to charge trapping when used with high x-ray fluxes, and periodically turning off the high voltage helps to clear the trapped charge.

HV_MODE $(P)$(R)HVMode, $(P)$(R)HVMode_RBV bo, bi

High voltage state. Choices are:

  • 0 - “Off”
  • 1 - “On”

This record turns the high voltage off and on. If HVMode is Auto then the high voltage will be turned on during an acquisition even if HVState is off.

HV_STATE $(P)$(R)HVState, $(P)$(R)HVState_RBV bo, bi
The high voltage value that will be applied to the detector when HVState=On or when HVMode=Auto and an acquisition is in progress. The allowed range is 0 to 400 volts. HV_VALUE $(P)$(R)HVValue, $(P)$(R)HVValue_RBV ao, ai
The actual high voltage currently being applied to the detector. HV_ACTUAL $(P)$(R)HVActual_RBV ai
The actual high voltage current. HV_CURRENT $(P)$(R)HVCurrent_RBV ai
Threshold parameters
Requested threshold energy in keV. There are 4 energy thresholds. The threshold energies are controlled by a single high-resolution register (VThMax) with values from 1500 to 2200, and 4 low-resoltion registers with values from 0 to 31. The driver attempts to set Threshold1 as closely as possible to the requested value by changing both VThMax and the low-resolution register. Thresholds 2-4 are then set as closely as possible to their requested values using only the low-resolution registers. THRESHOLDN (N=1-4) $(P)$(R)ThresholdN (N=1-4), $(P)$(R)ThresholdN_RBV (N=1-4) ao, ai
Actual threshold energy in keV. This will be as close as possible to the requested value, subject to the constraints and algorithm explained above. THRESHOLD_ACTUALN (N=1-4) $(P)$(R)ThresholdActualN_RBV (N=1-4) ai
This is only used on the PIII ASIC. It sets the threshold in keV where the PIII will consider a photon “hit” to have ocurred. This threshold is mainly intended for use when CountMode=NPI or NPISUM. However, even when CountMode=Normal it must always be set to a value less than Threshold 1. HIT_THRESHOLD $(P)$(R)HitThreshold, $(P)$(R)HitThreshold_RBV ao, ai
Actual hit threshold energy in keV. This will be as close as possible to the requested value, subject to the constraints and algorithm explained above. HIT_THRESHOLD_ACTUAL $(P)$(R)HitThresholdActual_RBV ai
External sync parameters

Polarity of the Sync In signal. Choices are:

  • 0 - “Pos.”
  • 1 - “Neg.”
SYNC_IN_POLARITY $(P)$(R)SyncInPolarity, $(P)$(R)SyncInPolarity_RBV bo, bi

Polarity of the Sync Out signal. Choices are:

  • 0 - “Pos.”
  • 1 - “Neg.”
SYNC_OUT_POLARITY $(P)$(R)SyncOutPolarity, $(P)$(R)SyncOutPolarity_RBV bo, bi

Function of the Sync Out signal. Choices are:

0 - “Shutter”
The Sync Out signal is high while the detector is collecting.,
1 - “Read done”
The Sync Out signal outputs a pulse when readout is complete.
2 - “Read”
The Sync Out signal is high while the detector is reading out.
SYNC_OUT_FUNCTION $(P)$(R)SyncOutFunction, $(P)$(R)SyncOutFunction_RBV mbbo, mbbi
Data collection status parameters
The number of colors collected so far for the current image. COLORS_COLLECTED $(P)$(R)ColorsCollected_RBV longin
The number of UDP buffers (images) read by the UDP listener thread for the current acquisition. UDP_BUFFERS_READ $(P)$(R)UDPBuffersRead_RBV longin
The maximum number of UDP buffers (images) for UDP listener thread. This is set at startup. UDP_BUFFERS_MAX $(P)$(R)UDPBuffersMax_RBV longin
The number of free UDP buffers (images). UDP_BUFFERS_FREE $(P)$(R)UDPBuffersFree_RBV longin
The speed with which the last UDP buffer was received (MB/s). UDP_SPEED $(P)$(R)UDPSpeed_RBV ai
Calibration and reset parameters
Sends a command to the detector to perform an autocalibration. The detector makes adjustments to achieve uniform pixel response. This operation must be performed at least once after the detector is power-cycled, and whenever necessary as the chip temperature and/or supply voltages may drift with time and environmental conditions. If autocalibration has not been performed then there will be many “hot” (non-zero) pixels in the image with no x-rays. AUTO_CALIBRATE $(P)$(R)AutoCalibrate, $(P)$(R)AutoCalibrate_RBV bo, bi
Writing 1 to this record sends a command to reset detector to its initial state. This causes the sockets to disconnect, and it takes about 30 seconds for the system to recover. Once it recovers the driver then sends commands to set all of the programmable parameters (thresholds, cooling, high voltage, etc.) to the current values in the EPICS output records. When the system is available again SystemReset record will go back to 0. SYSTEM_RESET $(P)$(R)SystemReset, $(P)$(R)SystemReset_RBV bo, bi

Unsupported standard driver parameters

The Pixirad driver does not support the following standard driver parameters:

  • Number of exposures per image (ADNumExposures)
  • Gain (ADGain)
  • Region to read out or binning (ADMinX, ADMinY, ADSizeX, ADSizeY, ADBinX, ADBinY, ADReverseX, ADReverseY)
  • Data type (NDDataType)


The Pixirad driver is created with the pixiradConfig command, either from C/C++ or from the EPICS IOC shell.

int pixiradConfig(const char *portName, const char *commandPortName,
                 int dataPortNumber, int statusPortNumber, int maxDataPortBuffers,
                 int maxSizeX, int maxSizeY,
                 int maxBuffers, size_t maxMemory,
                 int priority, int stackSize)

For details on the meaning of the parameters to this function refer to the detailed documentation on the pixiradConfig function in the pixirad.cpp documentation and in the documentation for the constructor for the pixirad class.

Example st.cmd startup file

There an example IOC boot directory and startup script provided with areaDetector: Example st.cmd Startup File.

MEDM screens

The following shows the MEDM screen that are used to control the Pixirad detector. Note that the general purpose screen ADBase.adl can be used, but it exposes many controls that are not applicable to the Pixirad, and lacks some fields that are important for the Pixirad.

pixirad.adl is the main screen used to control the Pixirad driver.

Performance measurements

The following measurements were done to demonstrate the performance that can be obtained with the areaDetector Pixirad driver. The timings were done by measuring the frequency of the Sync Out signal on an oscilloscope. The SyncOutFunction was “Shutter” and 1000 frames were collected.

FrameType AcquireTime Frames/sec
1 color low 0.02 35.7
1 color low 0.01 57.4
1 color low 0.01 57.4
1 color low 0.005 83.3
1 color low 0.001 125.0
1 color DTF 0.02 47.3
1 color DTF 0.01 93.8
1 color DTF 0.005 143.0
1 color DTF 0.001 143.0
2 color 0.02 28.5
2 color 0.01 41
2 color 0.005 52
2 color 0.001 66
2 color DTF 0.02 23.4

The measurements above were made with TriggerMode=Internal. Additional measurements made using TriggerMode=External showed that the maximum frame rate was the same as that shown in the table, i.e. as soon as the external trigger frequency exceeded this value the detector ignored every second external trigger pulse.

The data above show that the detector overhead is about 7.5 ms in “1 color low” mode. In “1 color DTF” mode the overhead is about 0.7 ms, but with a minimum frame period of 7 ms. In “2 color” mode the overhead is 15 ms, or 7.5 ms per image, the same as in “1 color low”. In “4 color” mode two exposures are required. The total time is equal to AcquireTime*2 + 0.0075*4, so again the overhead is about 7.5 ms per image.


The following are some current restrictions of the Pixirad driver due to bugs in the Pixirad firmware:

  • The detector does not accept a high-voltage value unless 2 different values are sent to it after power-cycle or system reset. The areaDetector driver works around this problem by sending the requested voltage minus 1.0 and then the requested voltage when the IOC starts up, and also after a system reset. However, it appears that even this is sometimes not sufficient. To see if this is a problem set the HVState to On and see if the HVActual is close to the requested value. If it is not, for example if it is only 12V or so, then change the HVValue to something else and then back to the requested value. This should fix the problem.
  • At high frame rates there can be occasional loss of UDP packets. This will result in an error message on the IOC console, UDPBuffersRead will be fewer than NumImages, and the Acquire PV will not go back to 0. If this happens then press the Stop button (setting Acquire to 0). If a DTF sequence was being acquired then sometimes all subsequent images will be corrupted, and it will be necessary to restart the EPICS IOC.
  • If a long acquisition sequence is started it is not currently possible to make the detector abort it. There is a command that can be sent to the detector to abort an acquisition, but unfortunately this causes the detector to malfunction for subsequent acquisitions.
  • The detector does not send UDP status packets during acquisition. If the HVMode=Auto and HVState=Off, which is a normal way to operate, then the HVActual readback will be 0 during acquisition, because no status update happens after the high voltage is automatically turned on at the start of acquisition. Immediately upon completion of acquisition the status updates begin, and a few non-zero values will be observed as the high voltage ramps down. This is normal, but it may cause confusion, since it appears that the high voltage has not been turned on during acquisition, when actually it was.