| At line 4 added 13 lines. |
| + Single tube counters and monitors are handled through the EL737 counter box. This seems simple |
| + but the EL737 counter box has some quircks. |
| + |
| + * Quite some confusion comes from the fact that meaningful cables from monitors and counters are |
| + plugged into the counter box and thus mapped to an integer id in the counter box. These interger IDs are |
| + directly mapped into SICS. Replugging at the counter box is easy. With the effect that it is never |
| + clear which monitor is which in SICS. |
| + * Counting on preset monitor works only on the monitor which is plugged into channel 1 of the counter box. |
| + * The EL737 counter box also detects the NoBeam condition through a monitor threshold. |
| + |
| + The EL737 counter box has in interesting output: this is the TTL trigger signal if data acquisition |
| + is active or not. |
| + |
| Line 7 was replaced by lines 20-87 |
| - !! Time-Of-Flight Data Acquisition |
| + In order to understand neutron DAQ with area detectors let us follow the path of a detected neutron through |
| + the system. |
| + |
| + # The neutron causes a charge on one of the detector wires |
| + # After preamplification and some signal processing the neutron causes a pulse on some wire |
| + # An electronic box called the Multi Detector Interface (MDI) detects the pulse and encodes it into a |
| + message to be sent through a fibre optic link. The neutron event message sent consists of: |
| + ## A message header identifying the message type |
| + ## The status of various sync bits. These are external inputs to the MDI. An important sync bit is the |
| + input from the EL737 counter box if data acquisition is active or not. |
| + ## The position information of the detected neutron event. |
| + # The fibre optic link transports the neutron event message to the histogram memory. |
| + # The histogram memory (HM) is indeed a computer. In the case of SINQ a MEN A12 VME PowerPC board running |
| + realtime linux. This HM computer runs two main tasks: a server task is responsible for configuring the |
| + HM and for communicating with then outside world which consists mostly of SICS. This server task is |
| + actually a WWW-server. Then there is the histogramming task: this task continuously reads the fibre optic |
| + link and analyizes the neutron event messages. It checks if the events are OK and accumulates them in |
| + the appropriate bins according to the position information in an in memory representation of the detector. |
| + # SICS communicates with the HM, configures it, starts and stops DAQ and download and saves the data to file. |
| + |
| + Even with an area detector an EL737 single counter box is necessary. This box handles the monitors and |
| + through its TTL output triggers neutron data acquisition. |
| + |
| + Just another time, to make the triggering process crystal clear. When data acquisition becomes active, |
| + the EL737 counter box sets its TTL output. This is fed into the MDI and is represented is one of the sync |
| + bits of the neutron event message. The histogramming task tests both the event type and the presence |
| + of the sync bit against a mask. Only matching neutron events with appropriate sync bits are histogrammed. |
| + |
| + The question may arise why this is so important. Now, SINQ is an unreliable source compared to a reactor. |
| + The beam may go down for milliseconds to days. But when the beam is down, the detector and electronics |
| + continue to produce noise. You do not want this noise on your data. Thus DAQ needs to be interrupted by |
| + way of the threshold in the counter box and the sync bit. |
| + |
| + |
| + !! Time-Of-Flight Neutron Data Acquisition |
| + |
| + In time-of-flight (TOF) neutron data acquisition not only the positional information of a neutron event is |
| + measured but also the time it needed to cover a certain distance before it arrived at the detector. This is |
| + a measure for the energy (speed) of the neutron. In order to do this, a starting point in time is needed |
| + when all neutrons started to travel towards the detector. At pulsed neutron sources this starting point in |
| + time is provided by the source. At SINQ it is provided by the chopper. This is a fast rotating disk which |
| + cuts the neutron beam into pulses by way of slits in the disk. |
| + |
| + TOF neutron DAQ is very similar to area detector data acquisition: |
| + # After some suitable electronics preprocessing the neutron event arrives at the MDI in a position encoded |
| + input channel. |
| + # The MDI maintains a counter which counts time. The time is the time since the last reset from the pulse |
| + generator. At SINQ, this reset signal is delivered by the chopper. |
| + # The MDI encodes the header, sync bits, time stamp and position information in a neutron event message. |
| + # The neutron event message is forwarded via the fibre optic link to the HM |
| + # The HM histograms the neutron event according to position and time stamp. |
| + |
| + Besides the normal sync inputs there are additional ones. Most notably an input from the chopper which |
| + indicates if the chopper is running at the desired speed and phase. If any of this is not the case, then |
| + the neutron event is useless. |
| + |
| + Another complication comes from the fact that most choppers have two slits (for symmetry) but deliver only |
| + one signal per chopper revolution. The signal needs to be duplicated. This is the purpose of the Emmenegger |
| + electronics. This is a rather obscure bit of hardware. Mr Emmenegger has left PSI a long time ago and |
| + no one knows any details about it anymore. |
| + |
| + Yet another complication comes from the fact that the data width for the time stamp is only 20 bits. In order |
| + to increase the available time resolution a delay time is subtracted in the MDI. This delay time is the |
| + time which the fastest interesting neutrons need to travel from the chopper to the detector. The delay |
| + time is set in the MDI by way of a RS232 connection from SICS. Unfortunately the implementation of the |
| + RS232 interface is dubious. RS232 is connected via a fibre optic link. Thus there is a fibre optic |
| + to RS232 converter. Some of those converters work, some not. One has to try out several ones when this |
| + link needs to be replaced until a converter is found which works. |
| At line 8 added 16 lines. |
| + Summing it up, the MDI in the TOF case needs the following inputs: |
| + # The position encoded inputs from the detector |
| + # A time reset signal from the chopper via the Emmenegger electronics |
| + # A sync input from the EL737 counter box which indicates wether DAQ is active or not |
| + # A sync input from the chopper which tells if the chopper is OK |
| + # A RS232 input to set the delay time from SICS |
| + |
| + Time-of-flight brings with it yet another complication: the spectrum of the incoming neutron beam is not |
| + flat. I.e. for different neutron energies there are different source intensities. In order to arrive at |
| + absolute (or relative) intensities in reduced data the scientist has to normalize against the source |
| + spectrum. To this purpose the monitors are passed into the MDI too and histogrammed as well. How this is |
| + done differs from TOF instrument to TOF instrument: At AMOR this data is handled as a different detector |
| + bank, on others the monitors are appended to the real data as extra detectors. |
| + |
| + |
| + |
| At line 10 added 42 lines. |
| + CCD cameras are like digital cameras in that incoming photons increment a charge in a detector pixel. |
| + After exposure, the charge map of the detector is read out. CCD detect photons: neutrons are detected by |
| + looking at scintillator screens which essentially converts neutrons into light. This explanation serves |
| + to give the background for the fact that the electronic noise suppression which works for normal |
| + detectors at the neutron event level does not work for CCDs. Thus one needs to rerun the |
| + exposure if SINQ goes down to long during an initial exposure. SICS checks for this and does the |
| + reexposure when necesary. |
| + |
| + Now, commercial CCDs come from companies living in the dark ages IT wise. Which means that they |
| + usually get deliverd with some windows system to run them. The better ones come with a Linux API |
| + in a library. This is what we use. On top of the library a litte WWW-server, ccdwww, is run which |
| + allows SICS to expose images and download data, configure the camera and such. This WWW-server is run |
| + at a separate computer. For a good reason, these CCD cameras require a special hardware connection. |
| + Which often fails. With a different computer the CCD camera computer can be restarted without |
| + having to take down the rest of the data acquisition system. |
| + |
| + |
| + !! Specialities at different Instruments |
| + |
| + ! AMOR |
| + |
| + At AMOR, the Emmenegger electronics died. Fortunately, the data for a full revolution of the chopper |
| + could be fitted into the 20 bit time stamps. Thus there are actually two pulses in the neutron event data. |
| + This is corrected for by a special histogramming process on the HM computer for AMOR which merges |
| + the two pulses. This makes the HM configuration for AMOR a little complicated. |
| + |
| + ! POLDI |
| + |
| + POLDI works with frame overlap. This means that the data from different neutron pulses overlap in the |
| + data. POLDI has a very complex chopper: the disk is split into four quadrants with each 8 slits. |
| + And there is only one reset signal from the chopper per revolution. Fortunately all data fits into the |
| + 20 bit time stamp. The raw data from the POLDI HM contains data from all four quadrants after each other |
| + in time. SICS sums the four quadrants together again. |
| + |
| + |
| + ! HRPT |
| + |
| + HRPT is the first instrument having the second generation data acquisition electronics. Namely the |
| + prototype. It replaces the MDI and front level electronics. This electronics has an own TCP/IP address |
| + and its own console. No documentation available, naturally. |
| + |
| + |
LIN