Table of ContentsTable of Contents 
Previous Chapter A single scan record supports a one dimensional scan. It is also possible to link scan records together to define multi-dimensional scans in a quite complex configuration.
Each scan record can control up to four "positioners" and acquire data from up to four process variables (typically detector data or measured positions of devices) during a scan. Two additional output variables can be defined to trigger other process variables (usually "detectors") between the positioning phase and the data acquisition phase. These outputs will be referred to as "detector triggers".
Although the typical use of a scan record is to move "positioners" and record "detector" data at each position, it can also be used for other applications. Any controllable device can be scanned through incremental values while recording data from any other process variables. For example, one of the "positioner" PVs could be used to vary the gain of a detector or the speed of a motor during a scan. Another example would be to use the scan record to vary several power supplies and record the beam position at each value of the supplies. In this context, the scan record becomes a general purpose "Vary w, x, y, z and record a, b, c, d" record. Therefore, throughout this document the word "positioner" and "controller" will be used synonymously. When referring to the data being recorded at each point, the word "detector" will be used.
All of the process variable names used to identify controllers, detectors, and detector triggers are specified using "reassignable links". This allows a scan to be configured on the fly. Scan parameters, including the names of controllers and detectors, can be saved and restored using the BURT.
NOTE: In this version, the PVs used in the "reassignable links" fields must reside in the same IOC.
In Figure 1, when the scan is initiated, the scan record commands several positioners (transform record and motor record) to move to their starting positions. The WAIT_1 record detects when all movement is complete and forces the SCAN record to process again. The SCAN record realizes that the positioning is complete and thus triggers the Detectors. The WAIT_2 record detects when data is valid and forces the SCAN record to process yet again. The SCAN record will then read the Detector Data and command the positioners to their next step. This will continue until the SCAN record has completed the appropriate number of steps. At the end of the scan, the SCAN record contains data arrays for each of the "detectors", as well as arrays that contain the positions to which each controller was commanded at each point. A simple x-y plot using this data will provide the detector data vs. position results.
Figure 1: Typical Database Support for a SCAN
Figure 2: A Two Dimensional Scan Implementation
To initiate a scan, the SCAN_Y record is commanded to begin. It commands its "positioners" to the specified starting point. The WAIT_1 record detects when all motors are stopped and forces the SCAN_Y record to process again. The SCAN_Y record will now write to its "Detector Trigger", which in this case begins a scan of the SCAN_X record. The SCAN_X record will now go through its entire programmed scan, acquiring data from the detectors at each point.
When the SCAN_X record is complete, the WAIT_4 record will force the processing of SCAN_Y, which will increment the position of the y-controller and initiate the x scan once again.
This approach to configuring a two dimensional scan is very flexible. Note that to "test" the x scan, one could write to the "BeginScan" field of SCAN_X which would perform an entire x scan. Although the SCAN_Y record would get processed after the x scan was complete (via the WAIT_4 record), nothing would happen because it was not in the middle of a scan. In addition, any of the motors can be moved individually when a scan is not in process without any unexpected behavior (detectors would not be triggered unless the operator did it deliberately). One could even build a three dimensional scan by adding an additional scan record that initiates the y-scan after positioning a z-controller.
There are three modes for determining the desired value for the positioner. If a positioner is specified as "linear", its desired value is determined by using parameters such as start_position, step_increment, number_of_steps, and end_position. This mode will be discussed in more detail in the next paragraph. If a positioner is specified as "lookup", its next position is found in an array that has been loaded into the record prior to initiating a scan. If the positioner is specified as "on-the-fly", it is commanded to the end_position after the first data point is collected and not changed again for the duration of the scan.
A linear scan involving a single positioner is fully defined by three parameters, e.g., the start position, the step size, and the number of data points. A scan involving N controllers is defined by merely 2N+1 parameters, since the number of data points must be the same for all controllers. For the convenience of interactive users, and to support channel-access clients that define scans differently, the scan record provides for six redundant scan-definition parameters (for the first controller only): START, END, CENTER, WIDTH, STEP, and NPTS. The record calculates values for unspecified parameters so that the set is always self consistent, and it imposes an upper limit (MAXPTS) on NPTS. WIDTH may be negative.
There is no unique prescription for removing inconsistencies among redundant parameters, and no hardcoded set of preferences among parameters is likely to please everyone. Therefore, the scan record allows the user to "freeze" parameters, so that they will not be changed in the record's internal attempts to ensure self-consistency of the parameter set. Frozen parameters can be changed by the user and by any other client. It is the user's responsibility to ensure that frozen parameters do not prevent freely specifying unfrozen parameters. For example, if both STEP and NPTS are frozen, changes to WIDTH will be rejected. Similarly, if both START and CENTER are frozen, changes to END and WIDTH will have no effect. By default, START, STEP, and NPTS are frozen. When the record cannot adjust the parameters to be consistent, a flag is raised (ALRT) and a message reported (SMSG).
Although this approach may seem to present the user with an overwhelming number of choices, it should be noted that "by default" the user only has to enter START, STEP, and NPTS to fully define the scan of a controller. The "user interface" (usually medm or another CA client) need only present the user with these fields. However, by changing the "freeze flags" from the default and presenting the user with different fields to fill in, the scan can be defined in a completely flexible way. The result is that a simple scan can be defined easily, but advanced users are not limited in flexibility.
For those controllers defined as "linear", each desired position is placed in the "desired position" array that is used for lookup mode. Therefore, after the scan is complete, this array will always contain the sequence of desired positions to which the controller was commanded. These values will have been provided by the user for "lookup" mode and calculated by the scan record for "linear" mode. This array will contain no useful data if "on-the-fly" mode was used.
If neither Detector Trigger contains a valid PV, the scan record will skip this step and acquire the data immediately. Note that specifying a "Detector Trigger" requires the scan record to be processed an additional time each point.
For single dimension scans, the scan is complete when the ExecuteScan flag is set back to zero by the record processing routine. The application program can then read the position arrays and the data arrays (or have a monitor set on them so the record will "post_monitors" when complete).
For two dimension scans similar to Figure 2, the application program should read the arrays from the SCAN_X record after the completion of each x scan and correlate it to the current y controller information. This will allow the application program to display data after each x scan. The scan record will buffer the data for only one x scan, so the application must read the arrays before the next x scan is completed. If the scan is fast enough that this may prevent a problem, the application program can contribute to the wait record algorithm such that the y -controllers are not moved until the application program has completely read the previous SCAN_X data.
On slow scans, the application program may want to see the scan is process on a point by point basis. Therefore, the scan record will "post_monitors" on fields that it updates each point, but it will not post monitors faster than 20 times per second. If a scan is proceeding at a rate less than 20 points per second, every point will be posted. If a scan is proceeding at 100 steps per second, scalar values will be posted every 5th point (approx). In either case, the array data will contain every point at the completion of the scan. It is not recommended that the application program use the point to point data except for keeping the operator aware of the progress of the scan.
---------------------------------------------------------------
Field Type DCT Initial Access Modify Rec Proc PP
Monitor
---------------------------------------------------------------
VERS FLOAT No 1.0 Yes No No No
VAL DOUBLE No 0 Yes Yes No No
SMSG STRING No Null Yes Yes Yes No
CMND ENUM No 0 Yes Yes Yes No
ALRT UCHAR No 0 Yes No Yes No
RPVT NOACCESS No Null No No No No
MPTS SHORT Yes 100 Yes No No No
EXSC SHORT No 0 Yes Yes Yes No
PXSC UCHAR No 0 Yes No No No
NPTS SHORT Yes 100 Yes Yes Yes No
FPTS RECCHOICE Yes 1 Yes Yes No No
CPT SHORT No 0 Yes No Yes* No
PCPT SHORT No 0 Yes No No No
TOLP ULONG No 0 Yes No No No
P1PV STRING Yes Null Yes Yes No No
P2PV STRING Yes Null Yes Yes No No
P3PV STRING Yes Null Yes Yes No No
P4PV STRING Yes Null Yes Yes No No
R1PV STRING Yes Null Yes Yes No No
R2PV STRING Yes Null Yes Yes No No
R3PV STRING Yes Null Yes Yes No No
R4PV STRING Yes Null Yes Yes No No
T1PV STRING Yes Null Yes Yes No No
T2PV STRING Yes Null Yes Yes No No
D1PV STRING Yes Null Yes Yes No No
D2PV STRING Yes Null Yes Yes No No
D3PV STRING Yes Null Yes Yes No No
D4PV STRING Yes Null Yes Yes No No
P1DB NOACCESS No Null No No No No
P2DB NOACCESS No Null No No No No
P3DB NOACCESS No Null No No No No
P4DB NOACCESS No Null No No No No
R1DB NOACCESS No Null No No No No
R2DB NOACCESS No Null No No No No
R3DB NOACCESS No Null No No No No
R4DB NOACCESS No Null No No No No
T1DB NOACCESS No Null No No No No
T2DB NOACCESS No Null No No No No
D1DB NOACCESS No Null No No No No
D2DB NOACCESS No Null No No No No
D3DB NOACCESS No Null No No No No
D4DB NOACCESS No Null No No No No
P1NV LONG No 0 Yes Yes Yes No
P2NV LONG No 0 Yes Yes Yes No
P3NV LONG No 0 Yes Yes Yes No
P4NV LONG No 0 Yes Yes Yes No
R1NV LONG No 0 Yes Yes Yes No
R2NV LONG No 0 Yes Yes Yes No
R3NV LONG No 0 Yes Yes Yes No
R4NV LONG No 0 Yes Yes Yes No
T1NV LONG No 0 Yes Yes Yes No
T2NV LONG No 0 Yes Yes Yes No
D1NV LONG No 0 Yes Yes Yes No
D2NV LONG No 0 Yes Yes Yes No
D3NV LONG No 0 Yes Yes Yes No
D4NV LONG No 0 Yes Yes Yes No
P1SM RECCHOICE Yes 0 Yes Yes No No
P1SP FLOAT Yes 0 Yes Yes Yes No
P1FS RECCHOICE Yes 0 Yes Yes No No
P1SI FLOAT Yes 0 Yes Yes Yes No
P1FI RECCHOICE Yes 0 Yes Yes No No
P1CP FLOAT Yes 0 Yes Yes Yes No
P1FC RECCHOICE Yes 0 Yes Yes No No
P1EP FLOAT Yes 0 Yes Yes Yes No
P1FE RECCHOICE Yes 0 Yes Yes No No
P1WD FLOAT Yes 0 Yes Yes Yes No
P1FW RECCHOICE Yes 0 Yes Yes No No
P1DV FLOAT No 0 Yes No Yes* No
P1LV FLOAT No 0 Yes No No No
R1CV FLOAT No 0 Yes No Yes* No
R1LV FLOAT No 0 Yes No No No
R1DL FLOAT Yes 0 Yes Yes No No
P1EU STRING Yes 16 Yes Yes No No
P1HR FLOAT Yes 0 Yes Yes No No
P1LR FLOAT Yes 0 Yes Yes No No
P1PR SHORT Yes 0 Yes Yes No No
P1PA FLOAT ARRAY No Null Yes Yes Yes No
P2SM RECCHOICE Yes 0 Yes Yes No No
P2SP FLOAT Yes 0 Yes Yes Yes No
P2FS RECCHOICE Yes 0 Yes Yes No No
P2SI FLOAT Yes 0 Yes Yes Yes No
P2FI RECCHOICE Yes 0 Yes Yes No No
P2CP FLOAT Yes 0 Yes Yes Yes No
P2FC RECCHOICE Yes 0 Yes Yes No No
P2EP FLOAT Yes 0 Yes Yes Yes No
P2FE RECCHOICE Yes 0 Yes Yes No No
P2WD FLOAT Yes 0 Yes Yes Yes No
P2FW RECCHOICE Yes 0 Yes Yes No No
P2DV FLOAT No 0 Yes No Yes* No
P2LV FLOAT No 0 Yes No No No
R2CV FLOAT No 0 Yes No Yes* No
R2LV FLOAT No 0 Yes No No No
R2DL FLOAT Yes 0 Yes Yes No No
P2EU STRING Yes 16 Yes Yes No No
P2HR FLOAT Yes 0 Yes Yes No No
P2LR FLOAT Yes 0 Yes Yes No No
P2PR SHORT Yes 0 Yes Yes No No
P2PA FLOAT ARRAY No Null Yes Yes Yes No
P3SM RECCHOICE Yes 0 Yes Yes No No
P3SP FLOAT Yes 0 Yes Yes Yes No
P3FS RECCHOICE Yes 0 Yes Yes No No
P3SI FLOAT Yes 0 Yes Yes Yes No
P3FI RECCHOICE Yes 0 Yes Yes No No
P3CP FLOAT Yes 0 Yes Yes Yes No
P3FC RECCHOICE Yes 0 Yes Yes No No
P3EP FLOAT Yes 0 Yes Yes Yes No
P3FE RECCHOICE Yes 0 Yes Yes No No
P3WD FLOAT Yes 0 Yes Yes Yes No
P3FW RECCHOICE Yes 0 Yes Yes No No
P3DV FLOAT No 0 Yes No Yes* No
P3LV FLOAT No 0 Yes No No No
R3CV FLOAT No 0 Yes No Yes* No
R3LV FLOAT No 0 Yes No No No
R3DL FLOAT Yes 0 Yes Yes No No
P3EU STRING Yes 16 Yes Yes No No
P3HR FLOAT Yes 0 Yes Yes No No
P3LR FLOAT Yes 0 Yes Yes No No
P3PR SHORT Yes 0 Yes Yes No No
P3PA FLOAT ARRAY No Null Yes Yes Yes No
P4SM RECCHOICE Yes 0 Yes Yes No No
P4SP FLOAT Yes 0 Yes Yes Yes No
P4FS RECCHOICE Yes 0 Yes Yes No No
P4SI FLOAT Yes 0 Yes Yes Yes No
P4FI RECCHOICE Yes 0 Yes Yes No No
P4CP FLOAT Yes 0 Yes Yes Yes No
P4FC RECCHOICE Yes 0 Yes Yes No No
P4EP FLOAT Yes 0 Yes Yes Yes No
P4FE RECCHOICE Yes 0 Yes Yes No No
P4WD FLOAT Yes 0 Yes Yes Yes No
P4FW RECCHOICE Yes 0 Yes Yes No No
P4DV FLOAT No 0 Yes No Yes* No
P4LV FLOAT No 0 Yes No No No
R4CV FLOAT No 0 Yes No Yes* No
R4LV FLOAT No 0 Yes No No No
R4DL FLOAT Yes 0 Yes Yes No No
P4EU STRING Yes 16 Yes Yes No No
P4HR FLOAT Yes 0 Yes Yes No No
P4LR FLOAT Yes 0 Yes Yes No No
P4PR SHORT Yes 0 Yes Yes No No
P4PA FLOAT ARRAY No Null Yes Yes Yes No
D1CV FLOAT No 0 Yes No Yes* No
D1LV FLOAT No 0 Yes No No No
D1EU STRING Yes 16 Yes Yes No No
D1HR FLOAT Yes 0 Yes Yes No No
D1LR FLOAT Yes 0 Yes Yes No No
D1PR SHORT Yes 0 Yes Yes No No
D1DA FLOAT ARRAY No Null Yes No Yes No
D2CV FLOAT No 0 Yes No Yes* No
D2LV FLOAT No 0 Yes No No No
D2EU STRING Yes 16 Yes Yes No No
D2HR FLOAT Yes 0 Yes Yes No No
D2LR FLOAT Yes 0 Yes Yes No No
D2PR SHORT Yes 0 Yes Yes No No
D2DA FLOAT ARRAY No Null Yes No Yes No
D3CV FLOAT No 0 Yes No Yes* No
D3LV FLOAT No 0 Yes No No No
D3EU STRING Yes 16 Yes Yes No No
D3HR FLOAT Yes 0 Yes Yes No No
D3LR FLOAT Yes 0 Yes Yes No No
D3PR SHORT Yes 0 Yes Yes No No
D3DA FLOAT ARRAY No Null Yes No Yes No
D4CV FLOAT No 0 Yes No Yes* No
D4LV FLOAT No 0 Yes No No No
D4EU STRING Yes 16 Yes Yes No No
D4HR FLOAT Yes 0 Yes Yes No No
D4LR FLOAT Yes 0 Yes Yes No No
D4PR SHORT Yes 0 Yes Yes No No
D4DA FLOAT ARRAY No Null Yes No Yes No
T1CD FLOAT Yes 0 Yes Yes No No
T2CD FLOAT Yes 0 Yes Yes No No
---------------------------------------------------------------
* Monitors on these fields are not posted any faster than 20 Hz. Some changes may not be posted! Do not rely on these fields for step-to-step info.
---------------------------------------------------------------------------------------------
Name Summary Description
---------------------------------------------------------------------------------------------
VERS Code Version Reflects the version of scan record processing routines.
VAL Value Field Not Used.
SMSG State Message This field contains a message from the record alerting the
operator to any error conditions. It can be cleared by
writing a `0' to the CMND field.
CMND Command Field This field is used to send commands to the record. The
following commands are currently defined:
0 - Clear the State Message Field
1 - Execute a "dry run" scan and check the desired
positions against the control limits of the positioner(s).
ALRT Alert Field This field is set to one when an alert condition exists in the
record (e.g Scan parameters too constrained, defined scan
exceeds limits, scan aborted, etc). The cause of the alert
will be indicated in the State Message Field (SMSG).
RPVT Record Private Pointer to a structure that maintains information about the
scan record.
MPTS Maximum Number This field is used to specify the maximum number of
of Points points that will be used for any scan defined for this
record. This value is used to allocate memory for all the
array oriented fields, so the value will significantly affect
memory usage.
EXSC Execute Scan Writing a `1' to this field will initiate a scan. Writing a `0'
will abort the scan. The record will reset this field to `0'
when the scan is finished.
PXSC Previous Execute Status of the EXSC flag the last time the record was
Scan processed.
NPTS Number of Points This entry defines the number of points for the scan. If the
freeze flag is set, the record will not change this value in
its attempt to keep the scan definition parameters
consistent. This value is constrained to be less than or
equal to MPTS.
FPTS Freeze Flag
CPT Current Point This field contains the current point of an active scan. The
posting of events on this field is throttled to 20 Hz, so for
fast scans not every new value will be posted.
PCPT Previous Current This field contains the value of CPT that was last posted
Point (not every change is posted, see CPT desctiption).
TOLP Time of Last Tick count of last time monitors were posted on the
Posting "throttled" fields (e.g. CPT)
PnPV Positioner n Process (n=1-4): These fields contain the Process Variable names
Variable of the controllers that will be commanded during the scan.
PnNV Positioner n Name (n=1-4): These flags indicate if the ASCII string entered
Valid in PnPV was found to be an existing Process Variable.
PnDV Positioner n (n=1-4): These fields contain the desired value for each
Desired Value postioner for the current point in the scan. The posting of
events on these fields is throttled to 20 Hz, so for fast
scans not every "desired value" will be posted.
PnLV Positioner n Last (n=1-4): Last value `posted' for PnDv.
Value
RnPV Readback n (n=1-4): If specified, the scan record will read this
Process Variable "actual position" of the controller to confirm it has
reached its "desired position". If this value is within a
specified deadband of the desired position, the scan will
continue.
RnNV Readback n Name (n=1-4): These flags indicate if the ASCII string entered
Valid in RnPV was found to be an existing Process Variable.
RnCV Readback n (n=1-4) : These fields contain the current value for each
Current Value postioner readback for the current point in the scan. The
posting of events on these fields is throttled to 20 Hz, so
for fast scans not every new value will be posted.
RnLV Readback n Last (n=1-4): Last value `posted' for RnCV.
Value
RnDL Readback n Delta (n=1-4) : If RnPV is specified, the scan record will
confirm that the Read Back value is within this dead band,
or the scan will be halted.
PnSM Positioner n Step (n=1-4) : Indicates if the desired positions for a controller
Mode are to be calculated in a linear fashion (incremented by
step size), found in a "desired position array", or given a
final position command at the beginning of the scan (on-
the-fly).
PnPA Positioner n (n=1-4): This array contains the step positions for a
Position Array controller if it uses the "lookup" mode (see PnSM). This
array is filled in by the record during a scan if the
controller was in "linear" mode. If the number of steps
was less than MPTS, the remainder of the array is filled
with the data from the last step.
PnSP Positioner n Start (n=1-4): For controllers in the "linear" or "on-the-
Position fly" mode, this field specifies the start position for the
controller. If the freeze flag is set, the record will not
change this value in its attempt to keep the scan definition
parameters consistent.
PnFS Positioner n Freeze
Flag
PnSI Positioner n Step (n=1-4): For controllers in the "linear" mode, this field
Increment specifies the step increment. If the freeze flag is set, the
record will not change this value in its attempt to keep the
scan definition parameters consistent.
PnFI Positioner n Freeze
Flag
PnEP End Position (n=1-4): For controllers in the "linear" mode, this field
contains the last position to which the controller will be
commanded. If the freeze flag is set, the record will not
change this value in its attempt to keep the scan definition
parameters consistent.
PnFE Positioner n Freeze
Flag
PnCP Positioner n Center (n=1-4): For controllers in the "linear" mode, this field
Position may be used to define the center position of a scan. If the
freeze flag is set, the record will not change this value in
its attempt to keep the scan definition parameters
consistent.
PnFC Positioner n Freeze
Flag
PnWD Positioner n Width (n=1-4): For controllers in the "linear" mode, this field
may be used to define the width of the scan (the distance
from the start position to the finish position). If the freeze
flag is set, the record will not change this value in its
attempt to keep the scan definition parameters consistent.
PnFW Positioner n Freeze
Flag
PnLR Positioner n Low (n=1-4): These are user configurable fields to describe low
Range and high ranges for the positioner data (used by some
channel access clients).
PnHR Positioner n High
Range
PnPR Positioner n (n=1-4) : This is a user configurable fields to describe the
Precision display precision for the positioner data (used by some
channel access clients).
PnEU Positioner n (n=1-4) : This is a user configurable fields to describe the
Engineering Units units for the positioner data (used by some channel access
clients).
TnPV Detector Trigger n (n=1-2): These fields contain the Process Variable names
Process Variable of the "Detector Triggers" that are written to between the
positioning phase and the data acquisition phase of the
record.
TnNV Trigger n Name (n=1-2): These flags indicate if the ASCII string entered
Valid in TnPV was found to be an existing Process Variable.
TnCD Trigger n Command (n=1-2): This is the data that is written to the Detector
Trigger PV's.
DnPV Data n Process (n=1-4): These fields contain the Process Variable names
Variable of the data that will be recorded at each point within the
scan.
DnNV Data n Name Valid (n=1-4): These flags indicate if the ascii string entered in
DnPV was found to be an existing Process Variable.
DnCV Detector n Current (n=1-4): These fields contain the current value for each
Value detector for the current point in the scan. The posting of
events on these fields is throttled to 20 Hz, so for fast
scans not every new value will be posted.
DnLV Detector n Last (n-1-4): Last value `posted' for DnCV.
Value
DnDA Detector n Data (n=1-4): This array (length = MPTS) contains the detector
Array data for each point in the scan. If the number of steps was
less than MPTS, the remainder of the array is filled with
the data from the last step.
DnLR Detector n Low (n=1-4): These are user configurable fields to describe low
Range and high ranges for the detector data (used by some
channel access clients).
DnHR Detector n High
Range
DnPR Detector n (n=1-4): This is a user configurable fields to describe the
Precision display precision for the detector data (used by some
channel access clients).
DnEU Detector n (n=1-4): This is a user configurable fields to describe the
Engineering Units units for the detector data (used by some channel access
clients).
PnDB Positioner n dbAddr (n=1-4): Pointer to the dbAddr structure of the PV
entered in PnPv. If PV is not found, the value is NULL.
RnDB Readback n dbAddr (n=1-4): Pointer to the dbAddr structure of the PV
entered in RnPv. If PV is not found, the value is NULL.
TnDB Trigger n dbAddr (n=1-2): Pointer to the dbAddr structure of the PV
entered in TnPv. If PV is not found, the value is NULL.
DnDB Detector n dbAddr (n=1-4): Pointer to the dbAddr structure of the PV
entered in DnPv. If PV is not found, the value is NULL.
---------------------------------------------------------------------------------------------
Table of Contents 
Next Chapter