Specifications

Section 5. Measurement and Control Peripherals

5-2

5.4.1 Binary Control

5.4.1.1 Digital I/O Ports

Each of eight digital I/O ports (C1 - C8) can be configured as an output port

and set low (0 V) or high (5 V) using the PortSet() or WriteIO() instructions.

A digital output port is normally used to operate an external relay driver circuit

because the port itself has very limited drive capability (2.0 mA minimum at

3.5 V).

5.4.1.2 Switched 12 V Control

The SW-12 port can be set low (0 V) or high (12 V) using the PortSet() or

SW12() instructions. The port is often used to control low power devices such

as sensors that require 12 V during measurement. Current sourcing must be

limited to 900 mA or less at 20°C.

A 12V switching circuit, driven by a digital I/O port, is also available from

Campbell Scientific.

The SW-12 supply is unregulated and can supply up to 900 mA

at 20°C and up to 630 mA at 50°C. A resettable polymeric fuse

protects against over-current. Reset is accomplished by

removing the load or turning off the SW-12 for several seconds.

5.4.1.3 Relays and Relay Drivers

Several relay drivers are manufactured by Campbell Scientific. Contact a

Campbell Scientific applications engineer for more information, or get more

information at www.campbellsci.com.

Compatible, inexpensive and reliable single-channel relay drivers for a wide

range of loads are available from various electronic vendors such as Crydom,

Newark, Mouser, etc.

5.4.1.4 Component Built Relays

FIGURE 5.4-1 shows a typical relay driver circuit in conjunction with a coil

driven relay which may be used to switch external power to some device. In

this example, when the control port is set high, 12 V from the datalogger passes

through the relay coil, closing the relay which completes the power circuit to a

fan, turning the fan on.

In other applications it may be desirable to simply switch power to a device

without going through a relay. FIGURE 5.4-2 illustrates a circuit for

switch

ing external power to a device without going through a relay. If the

peripheral to be powered draws in excess of 75 mA at room temperature (limit

of the 2N2907A medium power transistor), the use of a relay (FIGURE 5.4-1)

uld be required.

NOTE