AM16/32B Relay Multiplexer 19
8.1.4
Mixed sensor types
In applications where sensor types are mixed, experienced programmers can create multiple
configurations, though it is preferred to use multiple multiplexers for these situations. When
programming for mixed sensors on a single AM16/32B, it is especially important to verify that
each measurement is reasonable. Consult Campbell Scientific for application assistance when it is
necessary to multiplex markedly different sensor types in an application.
8.2
General measurement considerations
8.2.1
Long cable lengths
Longer sensor-to-AM16/32B cables result in greater induced and capacitively coupled voltages
(cross talk) between cable wires. It may also be necessary to program a delay within the
measurement instruction to allow time for wire capacitances to discharge after advancing a
channel, before the measurement is made. This can be done by increasing the
Delay
parameter
in the
PulsePort()
instruction or by adding a
Delay()
instruction after the
PulsePort()
instruction. A delay of 20 ms or more is recommended.
8.2.2
Completion resistors
In some applications, it is advantageous to place completion resistors at the AM16/32B terminal
strips. Certain sensors specific to the use of multiplexers are available from Campbell Scientific.
Examples include soil moisture probes and thermistor probes.
8.2.3
Contact degradation
Once excitation in excess of 30 mA has been multiplexed, that channel relay contacts are
unsuitable for further low voltage measurement. To prevent undue degradation, it is advisable to
reserve certain channels for sensor excitation and use other channels for sensor signals.
Refer to
(p. 2) for more information on contact degradation.
