Scaler Record

Overview
Field Descriptions
Restrictions

Overview

The scaler record provides support for a bank of up to 64 32-bit counters (scaler channels) controlled by a common start/stop mechanism.

Channel Types

If the hardware is capable of preset-counter operation, the record divides scaler channels into two groups according to the values of each channel’s gate-control field (Gn):

Simple counters (Gn = “N”): Zeroed immediately before counting begins, and count upwards.
Preset counters (Gn = “Y”): Simple counters that will not count higher than a preset value. The first preset counter to reach its preset value stops all counters.

The hardware may actually implement a preset counter by loading in the preset value and counting down to zero. If so, device support is expected to maintain the fiction that all counters count up from zero. Hardware that does not support preset counting (e.g., Joerger VS) will have Gn forced to “N” by device support.

Channel 1 and Time Base

Scaler channel 1 receives special treatment: it is presumed to count a reference-frequency clock signal and its value indicates the time interval during which counting was enabled. The fields T (time), TP (time preset), S1 (channel 1 value), PR1 (channel 1 preset), and FREQ (clock frequency) satisfy:

T  = S1  / FREQ
TP = PR1 / FREQ

Hardware Time Base Management

The manner in which the time base is managed differs by hardware type:

Joerger VSC scalers – The user must connect a clock signal to the channel-1 input and set FREQ to its frequency. The 10 MHz signal available on the front panel is normally used for this purpose.

SIS3801 scalers – The hardware connects an internally generated 25 MHz clock to channel 1 via a software switch. Any external signal connected to channel 1 will be ignored. It is possible to use an externally generated clock; in any case the user must ensure FREQ contains the correct value. The SIS3801 does not support hardware presets. Presets are emulated using a FIFO that buffers counts in 0.01 s dwell periods. The driver reads the FIFO at the scaler polling rate (RATE) and compares total counts against presets. Counting may extend up to 0.01 s beyond the requested time, but all channels count for exactly the same period, so counts per second remain accurate.

SIS3820 scalers – The hardware connects an internally generated 50 MHz clock to channel 1 via a software switch. The SIS3820 supports hardware presets, but with a limitation: there is only a single preset register for channels 1-16 and a second for channels 17-32. Only one channel from each group can serve as a preset at the same time. The driver uses the highest numbered channel in each group that has a non-zero preset (PRn) and active gate (Gn = “Y”). For example, if PR1, PR3, PR18, and PR22 are all non-zero with their gates active, counting stops when S3 = PR3 or S22 = PR22, whichever comes first.

Joerger VS scalers – This hardware does not implement preset counters. An internal 16-bit gate-time counter sets the counting time. Device support chooses a reference frequency and configures the hardware to generate it, then sets and posts the FREQ field accordingly. The user must still connect the reference signal to the channel-1 input so that the record and clients can treat channel 1 as the time base.

Counting Operation

The user sets FREQ to the frequency of the clock signal input to scaler channel 1 (Joerger VSC, SIS3801, and SIS3820 only).
The user sets TP (time preset) and/or PRn (preset value for channel n). For Joerger VS scalers, only TP or PR1 may be set.
The user sets CNT to “Count” (1), initiating counting.
Counting continues until any preset counter reaches its preset value, at which point the record resets CNT to “Done” (0). The user may also manually set CNT to “Done” to stop counting at any time.

Background Counting (AutoCount)

The scaler record supports two counting modes: normal and background (AutoCount).

Normal counting is controlled by the CNT field, the time preset, and possibly other presets as described above. Background counting is controlled by the CONT (“OneShot/AutoCount”) field and is intended to give the user continuous updates whenever the scaler would otherwise be idle.

When CONT = “AutoCount” (1) and no normal counting operation is in progress, the scaler waits for the AutoCount delay (DLY1), counts for the AutoCount period (TP1), and displays the result. If TP1 is less than 0.001 s, the normal preset criteria are used instead. If the AutoCount Display Rate (RAT1) is greater than zero, scaler values are displayed at that rate while background counting is in progress.

Background counting is interrupted immediately when a normal counting operation starts. After normal counting finishes, the results are held for ten seconds before background counting resumes. This hold period can be changed by setting the variable scaler_wait_time to the desired number of seconds, either at the IOC console or in the startup script.

Background counting never affects the state of the CNT field, so this field can always be used to determine when a normal counting operation has begun and finished.

Record Processing

The scaler record is unlike most other EPICS records in that its processing is neither “synchronous” nor “asynchronous” as these terms are used in the EPICS Application Developer’s Guide. The PACT field is always FALSE after record processing has completed, even though the scaler may still be counting. This means the record always responds to Channel Access puts, and counting can be stopped at any time. The record’s forward link is not executed until the scaler has stopped counting.

Field Descriptions

In addition to fields common to all record types (see Fields Common to All Record Types) the scaler record has the fields described below.

In the Access column: R = read only, R/W = read and write, R/W* = read and write where writing triggers record processing when SCAN is “Passive”, r = read only, not posted.

Control/status fields

Field	Access	Type	Choices	Description
CNT	R/W*	RECCHOICE	0: Done, 1: Count	Set to “Count” to start counting. When a preset is reached, counting stops, this field resets to “Done”, and the forward link fires. Setting to “Done” while counting is in progress stops counting and reports accumulated counts.
CONT	R/W*	RECCHOICE	0: OneShot, 1: AutoCount	Set to “AutoCount” to enable background counting. See also DLY1, RAT1, and TP1.
G1…G64	R/W	RECCHOICE	0: N, 1: Y	Gate control for each channel. Determines whether the channel is a simple scaler (N) or preset scaler (Y). Setting Gn to Y ensures PRn is nonzero (defaults to 1000). Setting PRn to a positive value automatically sets Gn to Y. Device support forces Gn to N on hardware that does not support presets.
PR1…PR64	R/W	ULONG		Preset values. If Gn = Y, counting stops when Sn reaches PRn. If Gn = N, PRn is ignored. Setting PRn to a positive value automatically sets Gn to Y.
TP	R/W	DOUBLE		Time preset in seconds. Acts as a proxy for PR1: changing TP sets PR1 = TP * FREQ.
TP1	R/W	DOUBLE		AutoCount time preset. Specifies the background-counting period in seconds.
DLY	R/W	FLOAT		Delay in seconds after CNT is set to “Count” before counting actually begins.
DLY1	R/W	FLOAT		AutoCount delay in seconds between successive background-counting periods.
RATE	R/W	FLOAT		Display update rate in Hz (0-60) during normal counting. If zero, counts are displayed only after counting stops.
RAT1	R/W	FLOAT		Display update rate in Hz (0-60) during background counting. If zero, counts are displayed only after background counting stops.

Data fields

Field	Access	Type	Description
FREQ	R/W	DOUBLE	Clock frequency in Hz for the signal counted by scaler channel 1. Used to convert between time values (T, TP) and channel 1 values (S1, PR1). The user must ensure this field is correct.
S1…S64	R	ULONG	Accumulated counts for each channel. Posted at RATE Hz while counting and once after counting stops.
T	R	DOUBLE	Elapsed time in seconds. Computed as S1 / FREQ whenever S1 changes.

Link fields

Field	Access	Type	Description
COUT	R/W	OUTLINK	Output link. CNT field value is written to this PV when a count sequence starts or finishes. Delayed by the DLY field.
COUTP	R/W	OUTLINK	Prompt output link. Same as COUT but not delayed by DLY – fires immediately when CNT changes.
OUT	R	OUTLINK	Output specification. Specifies the hardware to be controlled.

Miscellaneous fields

Field	Access	Type	Description
NM1…NM64	R/W	STRING	User-assigned names for individual scaler channels.
PREC	R/W	SHORT	Display precision. Number of digits to the right of the decimal point.
EGU	R/W	STRING	Engineering units string sent to Channel Access clients.
VERS	R	FLOAT	Code version number of the scaler record support.
NCH	R	SHORT	Number of channels supported by the hardware, as reported by device support.

Private fields

Field	Access	Type	Choices	Description
D1…D64	R/W	RECCHOICE	0: Up, 1: Dn	Count direction. The direction in which the hardware actually counts for channel n. These fields are for internal use and may become read-only in the future.
PCNT	R/W	RECCHOICE	0: Done, 1: Count	Previous value of the CNT field.
SS	r	SHORT		Scaler hardware state.
US	r	SHORT		User request state.
VAL	R/W	DOUBLE		Result. Set to the value of T and posted when counting is done, after all other fields have been posted.
RPVT	r	void *		Private pointer used internally by the record.

Restrictions

When AutoCount is enabled with a very short counting time (TP1) and delay time (DLY1), the scaler can swamp the network with data.