Precautions - Campbell SDM-SI01A Product Manual

For simplicity, except in sections specific to the SDM-SIO4A, only the SDM-SIO1A is referred to in
the text of this manual. All of the information in the manual applies to both products unless
otherwise noted.

2. Precautions

WARNING: Ensure that the 12V power supply for the module is switched off when
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connecting the power and ground wires to the SDM-SIO1A. When making connections to a
data logger, always ensure power to the data logger is switched off and connect the
ground (G) wire first.
WARNING: The RS-485 0V ground reference connection may be needed to ensure all units
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are referenced to a common ground voltage. This is more often needed with long cable
runs. It is advisable to check the difference in ground potential with some caution before
connecting any wires, to ensure the potential differences are reasonably small and
excessive current will not flow between the two ground wires. There is a current limiting
resistor fitted in the RG line in the SDM-SIO1A, but this will not be adequate in the event of
a serious ground fault, for example, the ground references being 240 V apart due to faulty
AC wiring. If a large potential difference is found, please seek the advice of a qualified
electrician before continuing with the installation
Figure 1-2. SDM-SIO1A Serial Input/Output Module
SDM-SIO1A and SDM-SIO4A Serial Input/Output Modules 2