Analog Input Record (ai)

This record type is normally used to obtain an analog value from a hardware input and convert it to engineering units. The record supports linear and break-point conversion to engineering units, smoothing, alarm limits, alarm filtering, and graphics and control limits.

Parameter Fields

The record-specific fields are described below, grouped by functionality.

Input Specification

These fields control where the record will read data from when it is processed:

Field Summary Type DCT Default Read Write CA PP
DTYP Device Type DEVICE Yes   Yes Yes No
INP Input Specification INLINK Yes   Yes Yes No

The DTYP field selects which device support layer should be responsible for providing input data to the record. The ai device support layers provided by EPICS Base are documented in the Device Support section. External support modules may provide additional device support for this record type. If not set explicitly, the DTYP value defaults to the first device support that is loaded for the record type, which will usually be the Soft Channel support that comes with Base.

The INP link field contains a database or channel access link or provides hardware address information that the device support uses to determine where the input data should come from. The format for the INP field value depends on the device support layer that is selected by the DTYP field. See Address Specification for a description of the various hardware address formats supported.

Units Conversion

These fields control if and how the raw input value gets converted into engineering units:

Field Summary Type DCT Default Read Write CA PP
RVAL Current Raw Value LONG No   Yes Yes Yes
ROFF Raw Offset ULONG No   Yes Yes Yes
ASLO Adjustment Slope DOUBLE Yes 1 Yes Yes Yes
AOFF Adjustment Offset DOUBLE Yes   Yes Yes Yes
LINR Linearization MENU (menuConvert) Yes   Yes Yes Yes
ESLO Raw to EGU Slope DOUBLE Yes 1 Yes Yes Yes
EOFF Raw to EGU Offset DOUBLE Yes   Yes Yes Yes
EGUL Engineer Units Low DOUBLE Yes   Yes Yes Yes
EGUF Engineer Units Full DOUBLE Yes   Yes Yes Yes

These fields are not used if the device support layer reads its value in engineering units and puts it directly into the VAL field. This applies to Soft Channel and Async Soft Channel device support, and is also fairly common for GPIB and similar high-level device interfaces.

If the device support sets the RVAL field, the LINR field controls how this gets converted into engineering units and placed in the VAL field as follows:

  1. RVAL is converted to a double and ROFF is added to it.
  2. If ASLO is non-zero the value is multiplied by ASLO.
  3. AOFF is added.
  4. If LINR is NO CONVERSION the units conversion is finished after the above steps.
  5. If LINR is LINEAR or SLOPE, the value from step 3 above is multiplied by ESLO and EOFF is added to complete the units conversion process.
  6. Any other value for LINR selects a particular breakpoint table to be used on the value from step 3 above.

The distinction between the LINEAR and SLOPE settings for the LINR field are in how the conversion parameters are calculated:

  • With LINEAR conversion the user must set EGUL and EGUF to the lowest and highest possible engineering units values respectively that can be converted by the hardware. The device support knows the range of the raw data and calculates ESLO and EOFF from them.
  • SLOPE conversion requires the user to calculate the appropriate scaling and offset factors and put them directly in ESLO and EOFF.

Smoothing Filter

This filter is usually only used if the device support sets the RVAL field and the Units Conversion process is used. Device support that directly sets the VAL field may implement the filter if desired.

The filter is controlled with a single parameter field:

Field Summary Type DCT Default Read Write CA PP
SMOO Smoothing DOUBLE Yes   Yes Yes No

The SMOO field should be set to a number between 0 and 1. If set to zero the filter is not used (no smoothing), while if set to one the result is infinite smoothing (the VAL field will never change). The calculation performed is:

where New Data was the result from the Units Conversion above. This implements a first-order infinite impulse response (IIR) digital filter with z-plane pole at SMOO. The equivalent continuous-time filter time constant τ is given by

where T is the time between record processing.

Undefined Check

If after applying the smoothing filter the VAL field contains a NaN (Not-a-Number) value, the UDF field is set to a non-zero value, indicating that the record value is undefined, which will trigger a UDF_ALARM with severity INVALID_ALARM.

Field Summary Type DCT Default Read Write CA PP
UDF Undefined UCHAR Yes 1 Yes Yes Yes

Operator Display Parameters

These parameters are used to present meaningful data to the operator. They do not affect the functioning of the record at all.

  • DESC is a string that is usually used to briefly describe the record.
  • EGU is a string of up to 16 characters naming the engineering units that the VAL field represents.
  • The HOPR and LOPR fields set the upper and lower display limits for the VAL, HIHI, HIGH, LOW, and LOLO fields.
  • The PREC field determines the floating point precision (i.e. the number of digits to show after the decimal point) with which to display VAL and the other DOUBLE fields.
Field Summary Type DCT Default Read Write CA PP
DESC Descriptor STRING [41] Yes   Yes Yes No
EGU Engineering Units STRING [16] Yes   Yes Yes No
HOPR High Operating Range DOUBLE Yes   Yes Yes No
LOPR Low Operating Range DOUBLE Yes   Yes Yes No
PREC Display Precision SHORT Yes   Yes Yes No

Alarm Limits

The user configures limit alarms by putting numerical values into the HIHI, HIGH, LOW and LOLO fields, and by setting the associated alarm severity in the corresponding HHSV, HSV, LSV and LLSV menu fields.

The HYST field controls hysteresis to prevent alarm chattering from an input signal that is close to one of the limits and suffers from significant readout noise.

The AFTC field sets the time constant on a low-pass filter that delays the reporting of limit alarms until the signal has been within the alarm range for that number of seconds (the default AFTC value of zero retains the previous behavior).

Field Summary Type DCT Default Read Write CA PP
HIHI Hihi Alarm Limit DOUBLE Yes   Yes Yes Yes
HIGH High Alarm Limit DOUBLE Yes   Yes Yes Yes
LOW Low Alarm Limit DOUBLE Yes   Yes Yes Yes
LOLO Lolo Alarm Limit DOUBLE Yes   Yes Yes Yes
HHSV Hihi Severity MENU (menuAlarmSevr) Yes   Yes Yes Yes
HSV High Severity MENU (menuAlarmSevr) Yes   Yes Yes Yes
LSV Low Severity MENU (menuAlarmSevr) Yes   Yes Yes Yes
LLSV Lolo Severity MENU (menuAlarmSevr) Yes   Yes Yes Yes
HYST Alarm Deadband DOUBLE Yes   Yes Yes No
AFTC Alarm Filter Time Constant DOUBLE Yes   Yes Yes No
LALM Last Value Alarmed DOUBLE No   Yes No No

Monitor Parameters

These parameters are used to determine when to send monitors placed on the VAL field. The monitors are sent when the current value exceeds the last transmitted value by the appropriate deadband. If these fields are set to zero, a monitor will be triggered every time the value changes; if set to -1, a monitor will be sent every time the record is processed.

The ADEL field sets the deadband for archive monitors (DBE_LOG events), while the MDEL field controls value monitors (DBE_VALUE events).

The remaining fields are used by the record at run-time to implement the record monitoring functionality.

Field Summary Type DCT Default Read Write CA PP
ADEL Archive Deadband DOUBLE Yes   Yes Yes No
MDEL Monitor Deadband DOUBLE Yes   Yes Yes No
ALST Last Value Archived DOUBLE No   Yes No No
MLST Last Val Monitored DOUBLE No   Yes No No
ORAW Previous Raw Value LONG No   Yes No No

Simulation Mode

The record provides several fields to support simulation of absent hardware. If the SIML field is set it is used to read a value into the SIMM field, which controls whether simulation is used or not:

  • SIMM must be zero (NO) for the record to request a value from the device support.
  • If SIMM is YES and the SIOL link field is set, a simlated value in engineering units is read using the link into the SVAL field, from where it will subsequently be copied into the VAL field.
  • If SIMM is RAW the SIOL link is still read into SVAL, but is then truncated and copied into the RVAL field. The “Units Conversion” process described above is then followed to transform the simulated raw value into engineering units.

The SIMS field can be set to give the record an alarm severity while it is in simulation mode.

Field Summary Type DCT Default Read Write CA PP
SIML Simulation Mode Link INLINK Yes   Yes Yes No
SIMM Simulation Mode MENU (menuSimm) No   Yes Yes No
SIOL Simulation Input Link INLINK Yes   Yes Yes No
SVAL Simulation Value DOUBLE No   Yes Yes No
SIMS Simulation Mode Severity MENU (menuAlarmSevr) Yes   Yes Yes No

Device Support Interface

The record requires device support to provide an entry table (dset) which defines the following members:

typedef struct {
    long number;
    long (*report)(int level);
    long (*init)(int after);
    long (*init_record)(aiRecord *prec);
    long (*get_ioint_info)(int cmd, aiRecord *prec, IOSCANPVT *piosl);
    long (*read_ai)(aiRecord *prec);
    long (*special_linconv)(aiRecord *prec, int after);
} aidset;

The module must set number to at least 6, and provide a pointer to its read_ai() routine; the other function pointers may be NULL if their associated functionality is not required for this support layer. Most device supports also provide an init_record() routine to configure the record instance and connect it to the hardware or driver support layer, and if using the record’s “Units Conversion” features they set special_linconv() as well.

The individual routines are described below.

Device Support Routines

long report(int level)

This optional routine is called by the IOC command dbior and is passed the report level that was requested by the user. It should print a report on the state of the device support to stdout. The level parameter may be used to output increasingly more detailed information at higher levels, or to select different types of information with different levels. Level zero should print no more than a small summary.

long init(int after)

This optional routine is called twice at IOC initialization time. The first call happens before any of the init_record() calls are made, with the integer parameter after set to 0. The second call happens after all of the init_record() calls have been made, with after set to 1.

long init_record(aiRecord *prec)

This optional routine is called by the record initialization code for each ai record instance that has its DTYP field set to use this device support. It is normally used to check that the INP address is the expected type and that it points to a valid device; to allocate any record-specific buffer space and other memory; and to connect any communication channels needed for the read_ai() routine to work properly.

If the record type’s unit conversion features are used, the init_record() routine should calculate appropriate values for the ESLO and EOFF fields from the EGUL and EGUF field values. This calculation only has to be performed if the record’s LINR field is set to LINEAR, but it is not necessary to check that condition first. This same calculation takes place in the special_linconv() routine, so the implementation can usually just call that routine to perform the task.

long get_ioint_info(int cmd, aiRecord *prec, IOSCANPVT *piosl)

This optional routine is called whenever the record’s SCAN field is being changed to or from the value I/O Intr to find out which I/O Interrupt Scan list the record should be added to or deleted from. If this routine is not provided, it will not be possible to set the SCAN field to the value I/O Intr at all.

The cmd parameter is zero when the record is being added to the scan list, and one when it is being removed from the list. The routine must determine which interrupt source the record should be connected to, which it indicates by the scan list that it points the location at *piosl to before returning. It can prevent the SCAN field from being changed at all by returning a non-zero value to its caller.

In most cases the device support will create the I/O Interrupt Scan lists that it returns for itself, by calling void scanIoInit(IOSCANPVT *piosl) once for each separate interrupt source. That routine allocates memory and inializes the list, then passes back a pointer to the new list in the location at *piosl.

When the device support receives notification that the interrupt has occurred, it announces that to the IOC by calling void scanIoRequest(IOSCANPVT iosl) which will arrange for the appropriate records to be processed in a suitable thread. The scanIoRequest() routine is safe to call from an interrupt service routine on embedded architectures (vxWorks and RTEMS).

long read_ai(aiRecord *prec)

This essential routine is called when the record wants a new value from the addressed device. It is responsible for performing (or at least initiating) a read operation, and (eventually) returning its value to the record.

… PACT and asynchronous processing …

… return value …

long special_linconv(aiRecord *prec, int after)

This optional routine should be provided if the record type’s unit conversion features are used by the device support’s read_ai() routine returning a status value of zero. It is called by the record code whenever any of the the fields LINR, EGUL or EGUF are modified and LINR has the value LINEAR. The routine must calculate and set the fields EOFF and ESLO appropriately based on the new values of EGUL and EGUF.

These calculations can be expressed in terms of the minimum and maximum raw values that the read_ai() routine can put in the RVAL field. When RVAL is set to RVAL_max the VAL field will be set to EGUF, and when RVAL is set to RVAL_min the VAL field will become EGUL.

The formulae to use are:

Note that the record support sets EOFF to EGUL before calling this routine, which is a very common case (when RVAL_min is zero).

Extended Device Support

…