.(34) Similarly Vrei (which gives i

.(34) Similarly Vrei (which gives information on battery status) will be sampled through the same analog input line PB1, if PA0 is now high while PA1 is low.

.(35) The other three analog input lines are not used here, but could be used for other environmental parameters such as ambient temperature and humidity.
.(36) An LED shows the status of the system. The LED is illuminated when measurements are being taken and while data is being transferred between the data acquisition system and the computer.
.(37) Line PA2 capable of sinking 20 mA current is used for directly driving this indicator LED. The status LED is lit when a logic 0 is placed on this line and is turned off by a logic 1.
(38)The SolData pyranometer is the most expensive component of the system, costing about U.S $300.
.(39) The rest of the data acquisition system components cost less than U.S. $60.
.(40) The sensor produces a
(41)transfer interface between the data acquisition system and the computer.

.(42) It is only powered when a computer is connected to the data acquisition system to retrieve the collected data.
.(43) Capacitors C3, C4, and C7 are provide power supply filtering to the ST6220, the 24C65 memory chip and the MAX232 respectively.
.(44) The reset pin on the microcontroller is active low and is held to a positive voltage by a 2.5 kil pull-up resistor.
.(45) A touch of the push button brings down the potential of the reset pin to ground thereby causing a system reset..(46) Capacitor C8 is used to eliminate debouncing of the push button which causes the microcontroller to reset many times.(47)Diode D1 is used for protecting the system against damage due to accidental battery polarity reversal.
(48)acquisition, digital, inter¬face, interrupt, microcontroller, programmable, serial.(49)horizontal surface. Monitoring as described herein deals with collection,(50)recording and transmission of the measured data to the computer for archival storage and off-site analysis of the data acquisition system performance..(51) This data is sampled and recorded as a function of lapsed time of the experiment..(52) This improved the quality of data since small errors are involved in digital data handling compared to conventional manual methods(53)This system is well suited for monitoring meteorological or environmental parameters at remote stations(54), particularly in developing countries where electrical power and telephone lines for telemetric data transfer are not readily available [5].].(55) One operator with one portable computer is all what is required to collect acquired data from many such systems scattered around an area of interest.
II. (muc II )HARDWARE DESIGN
(56)The data acquisition system developed is a compact (70 x 50 x 30 mm), low cost, 8-bit system with 8 analog input channels designed for automatic long-term data collection.. Fig.(57) 1 shows the block diagram of the basic elements of the design.(58)The system was designed to be versatile and all operations are under software control..(59) This will allow for future expansion or modifications without the need for major hardware changes..(60) The system is connected to a computer through the RS232 serial link to allow user communications and to download recorded data to the computer for subsequent analysis..(61) The main component of the data acquisition system is the ST62E20 microcontroller that is driven by an 8 MHz crystal oscillator.