Programmable logic control

The data logger can control instruments and devices such as:

• Controlling cellular modem or GPS receiver to conserve power.

• Triggering a water sampler to collect a sample.

• Triggering a camera to take a picture.

• Activating an audio or visual alarm.

• Moving a head gate to regulate water flows in a canal system.

• Controlling pH dosing and aeration for water quality purposes.

• Controlling a gas analyzer to stop operation when temperature is too low.

• Controlling irrigation scheduling.

Control decisions can be based on time, an event, or a measured condition. Controlled devices can be physically connected to C, U, or SW12 terminals. Short Cut A CRBasic programming wizard suitable for many data logger applications. Knowledge of CRBasic is not required to use Short Cut. has provisions for simple on/off control. Control modules and relay drivers are available to expand and augment data logger control capacity.

• C and U terminals are selectable as binary inputs, control outputs, or communications ports. These terminals can be set low (0 VDC) or high (3.3 or 5 VDC) using the PortSet() or WriteIO() instructions. See the CRBasic Editor help for detailed instruction information and program examples: https://help.campbellsci.com/crbasic/cr6/ . Other functions include device-driven interrupts, asynchronous communications and SDI-12 communications. The high voltage for these terminals defaults to 5 V, but it can be changed to 3.3 V using the PortPairConfig() instruction. A C or U terminal configured for digital I/O is normally used to operate an external relay-driver circuit because the terminal itself has limited drive capacity.

• SW12 terminals can be set low (0 V) or high (12 V) using the SW12() instruction (see the CRBasic help for more information).

The following image illustrates a simple application wherein a C or Uterminal configured for digital input, and another configured for control output are used to control a device (turn it on or off) and monitor the state of the device (whether the device is on or off).

(Click image to expand/collapse display)

In the case of a cell modem, control is based on time. The modem requires 12 VDC power, so connect its power wire to a data logger SW12 terminal. The following code snip turns the modem on for the first ten minutes of every hour using the TimeIsBetween() instruction embedded in an If/Then logic statement:

If TimeIsBetween (0,10,60,Min)Then    SW12(SW12_1,1,1) 'Turn phone on. Else    SW12(SW12_1,0,1) 'Turn phone off. EndIf