Campbell CDM-A100 Series, Инструкция по эксплуатации

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C-1
Appendix C. Calculating Network
Restrictions
This section shows some common questions asked while designing networks
with CDM-A100s and how to calculate their answers. For all examples, the
CDM-A108 or CDM-A116 is connected to a CR6 datalogger.
C.1 Example 1
a) How fast can a CDM-A116 make 32 single-ended measurements?
The time to make 32 single-ended measurements (which are measurements
without input or excitation reversal) is copied below from TABLE
8-1 ,
Multiplexed Analog Voltage Measurement Speed
(p. 26)
:
Measurement Time = repetitions
• �
T
settle
+
10
6
µs/s
f
N1
+ 184
µs
� + 31
µs
To make the fastest measurements possible, set f
N1
to its maximum (30000 Hz)
and T
settle
to its minimum (100 µs). With these inputs and 32 repetitions, the
fastest measurement time is calculated as 10186 µs.
Measurement Time = 32
• � 100
µs
+
10
6
µs/s
30000
Hz
+ 184
µs
� + 31
µs
= 10186 µs
The CR6 has a scan rate resolution of 1 ms, so the fastest scan rate possible in
this example is 11 ms.
b) Choosing a more practical scan rate of 20 ms, what is the longest cable
that could be used between the CDM-A116 and the CR6 to make these
32 measurements?
A scan rate of 20 ms (0.02 s) equals a measurement frequency of 50 Hz (1 ÷
0.02 s). Using Equation
8-1 , the kilobits of data generated each second is
Kilobits per second = (number of measurements) × (measurement frequency) ×
(0.064 kilobits per measurement)
= (32 measurements) × (50 Hz) × (0.064 kilobits per measurement)
= 102.4 kbps
In this case, the CPISpeed() instruction could be used to change the bit rate
from the 250 kbps default to 125 kbps, allowing a longer cable. Then, referring
to TABLE
8-2 , Network Topology, Bit Rate, and Cable Length
(p. 28)
, the
longest cable length possible is 1200 ft with the use of a termination plug, or
1000 ft without.