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Appendix C. Calculating Network
Restrictions
C.1 Example 1
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.
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
Measurement Time = repetitions • �T
To make the fastest measurements possible, set f
and T
to its minimum (100 µs). With these inputs and 32 repetitions, the
settle
fastest measurement time is calculated as 10186 µs.
Measurement Time = 32 • �100 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
longest cable length possible is 1200 ft with the use of a termination plug, or
1000 ft without.
10
:
(p. 26)
6
+
+ 184 µs� + 31 µs
µs/s
settle
f
N1
to its maximum (30000 Hz)
N1
10
6
+ 184 µs� + 31 µs
30000 Hz
µs/s
(p. 28)
, the
C-1
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