Campbell CR800 Series - 8 Operation; Measurements - Details; Time Keeping - Details; Time Stamps

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8. Operation
Related Topics:
• Quickstart (p. 35)
• Specifications (p. 91)
• Installation (p. 93)
• Operation (p. 311)
8.1 Measurements — Details
Related Topics:
• Sensors — Quickstart (p. 35)
• Measurements — Overview
(p. 64)
• Measurements — Details
(p. 311)
• Sensors — Lists
(p. 567)
Several features give the CR800 the flexibility to measure most sensor types.
Some sensors require precision excitation or a source of power. See Switched-
Voltage Output — Details
(p. 388).
8.1.1 Time Keeping — Details
Related Topics:
• Time Keeping — Overview (p. 65)
• Time Keeping — Details (p. 311)
—Measurement of time is an essential function of the CR800. Time
measurement with the on-board clock enables the CR800 to attach time stamps to
data, measure the interval between events, and time the initiation of control
functions.
8.1.1.1 Time Stamps
A measurement without an accurate time reference has little meaning. Data on
the CR800 are stored with time stamps. How closely a time stamp corresponds
to the actual time a measurement is taken depends on several factors.
The time stamp in common CRBasic programs matches the time at the beginning
of the current scan as measured by the real-time clock in the CR800. If a scan
starts at 15:00:00, data output during that scan will have a time stamp of 15:00:00
regardless of the length of the scan or when in the scan a measurement is made.
The possibility exists that a scan will run for some time before a measurement is
made. For instance, a scan may start at 15:00:00, execute time-consuming code,
then make a measurement at 15:00:00.51. The time stamp attached to the
measurement, if the CallTable() instruction is called from within the Scan() /
NextScan construct, will be 15:00:00, resulting in a time-stamp skew of 510 ms.