Campbell Scientific CR7 Measurement and Control System - Manual (Page 150)

SECTION 14. INSTALLATION
14-6
20 AWG wire. This transient protection is
useless if there is not a good connection
between the CR7 and earth ground.
All dataloggers in use in the field should be
grounded. A 12 AWG or larger wire should be
run from the grounding terminal on the left side
of the I/O Module (Figure OV1-1) to a grounding
rod driven far enough into the soil to provide a
good earth ground.
A modem/phone line connection to the CR7
provides another pathway for transients to enter
and damage the datalogger. The DC112
Modem has spark gaps on the phone lines. A
ground wire should be run between the ground
terminal on the modem and earth ground.
14.4.2 EFFECT ON MEASUREMENTS: COMMON
MODE RANGE
The effects that a difference in ground potential
between a sensor or signal conditioner and the
CR7 can have on a measurement were
discussed in Sections 7.2 and 13.2. These
sections stress that differential voltage
measurement gets rid of offset caused by a
difference in ground potential. However, in
order to make a differential measurement, the
inputs must be within the CR7 common mode
range of ±5V.
The common mode range is the voltage range,
relative to CR7 ground, within which both inputs
of a differential measurement must lie, in order
for the differential measurement to be made.
For example, if the high side of a differential
input is at 4V and the low side is at 3V relative
to CR7 ground, there is no problem, a
measurement made on the ±1.5V range would
indicate a signal of 1V. However, if the high
input is at 5.8V and the low input is at 4.8V, the
measurement cannot be made because the
high input is outside of the CR7 common mode
range.
Problems with exceeding common mode range
may be encountered when the CR7 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
CR7. 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 CR7 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 CR7 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.
14.5 USE OF DIGITAL CONTROL
PORTS FOR SWITCHING RELAYS
Each of the eight digital control output ports can
be set high or low by the PORT SET command
(Instruction 20). Because of current supply
limitations, a digital control output port normally
is used to operate an external relay driver.
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.
Figure 14.5-1 is a schematic representation of a
typical external coil driven relay configuration
which may be used in conjunction with one of
the CR7s digital control output ports. The
example shows a DC fan motor (typical of a
ventilated psychrometer) and 6V battery in the
circuit, but the configuration may be used for
other purposes. This particular configuration
has a coil current limitation of 75mA because of
the NPN Medium Power Transistors used (Part
No. 2N2222). CSI's Model A21REL-12 and A6
REL12 are 12 VDC Relay Controllers available
for use with the CR7 system.
FIGURE 14.5-1. Typical Connection for
Activating/Powering External Devices, Using a
Digital Control Output Port and Relay Driver.