Campbell Scientific CR9000X Measurement and Control System - Manual (Page 71)

Section 1. Installation
Sensors that have a floating output or are not referenced to ground through a
separate connection may need to have one side of the differential input
connected to ground to ensure the signal remains within the operational
voltage range.
Problems with exceeding the operational input voltage range may be
encountered when the CR9000X is used to read the output of external signal
conditioning circuitry if a good ground connection does not exist between the
external circuitry and the CR9000X. When operating where AC power is
available, it is not always safe to assume that a good ground connection exists
through the AC wiring. If a CR9000X is used to measure the output from a
laboratory instrument (both plugged into AC power and referencing ground to
outlet ground), it is best to run a ground wire between the CR9000X and the
external circuitry. Even with this ground connection, the ground potential of
the two instruments may not be at exactly the same level, which is why a
differential measurement is desired.
1.5 Use of Digital Control Ports for Switching Relays
The digital control outputs on the CR9060 Excitation Module and the I/O
channels on the CR9070/CR9071E Counter Timer Module may be used to
actuate controls, but because of current supply limitations, the output ports are
not used directly to drive a relay coil. Relay driver circuitry is used to switch
current from another source to actually power the relay. These relays may be
used for activating an external power source to run a fan motor or for altering
an external circuit as a means of multiplexing signal lines, etc. CSI's Model
A21REL-12 and A6 REL12 are Relay Controllers using a 12 VDC source for
switching the relays. Solid state relays that may be controlled with a 0-5 V
logic signal are also available for switching AC or DC power.
Figure 1.5-1 is a schematic representation of a typical external coil driven relay
configuration which may be used in conjunction with one of the CR9000Xs
digital control output ports. The example shows a DC fan motor and 12 V
battery in the circuit. This particular configuration has a coil current limitation
of 75 mA because of the NPN Medium Power Transistors used (Part No.
2N2222).
FIGURE 1.5-1. Typical connection for activating/powering external
devices, using a digital control output port and relay driver
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