Measurement Specialties
KPSI
®
Digital Series User’s Manual
Page 10
www.Meas-Spec.com
atttn
:
a
Where “a” is
sensor address
, “ttt” is
time (in seconds)
sensor needs to make the
measurement or process command, and “n” is the
number of data values
(limited
to 9) that may be fetched by the
Get Data
(“D0”) command when data is ready. In
this older form, the sensor responds a second time with its address “a” when the
data is ready (if “ttt” was not 000). This second response is called a
Service
Request
response. This format is generated by older
non-concurrent
measurement commands (see “M[v]” commands below) and other older
commands.
atttnn
Where “a” is
sensor address
, “ttt” is
time (in seconds)
sensor needs to make the
measurement or process the command and “nn” is the
number of data values
that
may be fetched by the
Get Data
(“D0”) command when data is ready. This
newer response format (in SDI Version 1.2 and later) does not generate the
second
Service Request
response and can return a larger number of values (“nn”)
limited to 99. This format is generated by a newer
concurrent
measurement
commands (see “C[v]” commands below).
If you issued the
Change Sensor Address
command or the
Identify
command described in the
previous sections, you already have some experience with using commands. There are other
commands available to make measurements, set the type of output units for the measurements,
perform special scaling of the measurements, do field calibration, and the like. The following
sections describe the commands by function.
M a k i n g a M e a s u r e m
e n t
There are two classes of measurement commands which will be referred to as
Start Non-
Concurrent Measurement
(“M[v]”) commands) and
Start Concurrent Measurement
(“C[v]”)
commands). In the first class the data recorder issues the measurement command and then
waits for the sensor to complete the measurement before continuing the data collection cycle.
Only one sensor can be accessed at a time and a maximum of nine parameters can be
returned. With Version 1.2 of the specification,
concurrent
measurements are permitted. With a
concurrent
measurement, the data recorder can request the sensor to take a measurement,
determine how long it will be until the sensor has a reading, and then continue making requests
to other sensors on the SDI-12 bus. This way, multiple sensors are taking measurements
concurrently. Once the measurement time for a sensor expires, the data recorder polls that
sensor for its data.