Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 86)

Section 7. Installation
86
Note Table Current Source and Sink Limits
(p. 84)
has more information on
excitation load capacity.
7.4.5.3 Continuous Unregulated (Nominal 12 Volt)
Voltage on the 12V terminals will change with CR800 supply voltage.
7.4.5.4 Switched Unregulated (Nominal 12 Volt)
The SW12 terminal is often used to control low power devices such as sensors
that require 12 Vdc during measurement. Current sourcing must be limited to 900
mA or less at 20°C. See table Current Source and Sink Limits
(p. 84).
Voltage on a
SW12 terminal will change with CR800 supply voltage. Two CRBasic
instructions, SW12() and PortSet(), control a SW12 terminal. Each instruction is
handled differently by the CR800. SW12() is a processing task. Use it when
controlling power to SDI-12 and serial sensors that use SDI12Recorder() or
SerialIn() instructions respectively. CRBasic programming using IF THEN
constructs to control SW12, such as when used for cell phone control, should also
use the SW12() instruction.
PortSet() is a measurement task instruction. Use it when powering analog input
sensors that need to be powered just prior to measurement.
A 12-Vdc switching circuit, designed to be driven by a digital I/O port, is
available from Campbell Scientific and is listed in the appendix Relay Drivers
(p.
541).
Note The SW12 terminal supply is unregulated and can supply up to 900 mA at
20°C. See table Current Source and Sink Limits
(p. 84).
A resettable polymeric
fuse protects against over-current. Reset is accomplished by removing the load or
turning off SW12 for several seconds.
7.5 Grounding
Grounding the CR800 with its peripheral devices and sensors is critical in all
applications. Proper grounding will ensure maximum ESD (electrostatic
discharge) protection and measurement accuracy.
7.5.1 ESD Protection
ESD (electrostatic discharge) can originate from several sources, the most
common, and most destructive, being primary and secondary lightning strikes.
Primary lightning strikes hit the datalogger or sensors directly. Secondary strikes
induce a voltage in power lines or sensor wires.
The primary devices for protection against ESD are gas-discharge tubes (GDT).
All critical inputs and outputs on the CR800 are protected with GDTs or transient
voltage suppression diodes. GDTs fire at 150 V to allow current to be diverted to
the earth ground lug. To be effective, the earth ground lug must be properly
connected to earth (chassis) ground. As shown in figure Schematic of Grounds
(p.
88),
power ground and signal grounds have independent paths to the ground lug.
Nine-pin serial ports are another path for transients. Communications paths, such
as telephone or short-haul modem lines, should be provided with spark-gap
protection at installation. Spark-gap protection is usually an option with these