Solartron Mobrey 3595 4C User Manual Download Page 74

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IMP Commands 3595 4C User Manual Part Two

1-22

35952350 (CH01/AA)

(Scan Period)

Where:

is a 4-byte floating-point number that defines the scan period in the range 0ms through

16777215ms.

Note:

This command does not apply to type 1D IMP devices.

Function:

The

command defines the period between start points of successive scans in the continuous

measurement mode and this allows scan data to be sent to the PC at a defined rate.

Defined scan periods are intended for an IMP in the fast scanning mode. (See entry for the

command.) For example, a 3595 1A IMP could be set to make a fast scan of 20

thermocouples every second, the duration of each scan being 300ms (FR2):

Scan Period, e.g. 1s

Actual scan time,
e.g. 300ms

Time

Trigger

Figure 1.1: Scan Period Command

Defining the scan period gives two advantages: (a) it provides a manageable amount of
useful data for the PC, and (b) the data becomes available at predictable intervals, thus
simplifying the reading of the data.

On power-up, the scan period is set to the default value of 0ms, which allows the IMP to
output scan data at the fastest rate possible. With this, however, the intervals at which data
becomes available are unpredictable and, if the PC is unable to cope with the large amount of
data produced, the system may hang. To make scanned data more manageable and
predictable, the scan period defined by the SP command should also allow the PC enough
time to process each block of scan data as it occurs. For a guide, the average scan times for
the various types of IMP and their measurement modes are listed in Table 1.10.

The scan period may be re-defined at any time. If a defined scan period is in operation, the
new period effectively merges with the old one. (See

note

on exception to this.) For

example, consider a scan with a defined period of 5 seconds that has been running for one
second. Commanding a new scan period of four seconds causes the next scan to start in
three seconds.

Conversely, if the new scan period is one second or less, the next scan starts immediately on
completion of the present scan. Note that a scan is never terminated by re-definition of the
scan period.

Note:

When a new scan period is loaded with the LO command (with continuous scanning),

the new period takes effect from the beginning of the next scan.

An IMP can not be made to output data faster than the inherent measurement rate. If the
defined scan period is less than the actual scan time, the IMP outputs the scan data at the
maximum uncontrolled rate.

The defined scan period is not effective for single scans. With the continuous mode
inoperative, scans start immediately on trigger. To ensure long term repeatability of the
defined scan period, the analogue type IMP has its’ internal calendar clock synchronised to
the time in the 4C Interface card. Synchronisation occurs every second. (Due to the
uncertainty of the clock in the PC/4C Interface card, a small number of scans may be lost or
gained over a 24-hour period. This number is

≤

2500

scan period

, where the scan period is

in milliseconds.)