Built-in test equipment: Vital parts of an AWS often include components whose faulty operation or failure would seriously degrade or render useless the principal output. The inclusion of circuits to monitor automatically these components’ status is an effective means of continuously controlling their performance during operation. Examples are: a power-failure detector which restarts the processor and continues the AWS function after a power failure; a “watchdog” timer to monitor the proper operation of microprocessors; and test circuits for monitoring the operation of station subsystems such as battery voltage and charger operation, aspirators (temperature and humidity screens), A/D converters, heaters, etc. Status information for quality-control and maintenance purposes. Guidance on preparing a functional specification for the AWS system is available in Part I of WMO (1997).
Parallel digital input/output: The total number of individual channels is mostly grouped in blocks of 8 out of 16 bits with extension possibilities. They are used for individual bit or status sensing or for input of sensors with parallel digital output (for example, wind vanes with Gray code output). Pulses and frequencies: The number of channels is generally limited . Typical sensors are wind speed sensors and (tipping-bucket) raingauges. Use is made of low- and high-speed counters accumulating the pulses in CPU memories. . Serial digital ports: These are individual asynchronous serial input/output channels for data communication with intelligent sensors. The ports provide conventional inter-device communications over short (RS232, several mets) to long (RS422/485, several kilomets). Different sensors or measuring systems on the same line and input port, each of the sensors addressed sequentially by means of coded words. for connecting the sensor to the data-acquisition electronics. Sensor cables: Electrical signals from the sensors entering a data-acquisition system might include unwanted noise. Whether this noise is troublesome depends upon the signal-to-noise ratio and the specific application. Digital signals are relatively immune to noise because of their discrete (and high-level) nature. In contrast, analogue signals are directly influenced by relatively low-level disturbances. The major noise transfer mechanisms include capacitive and inductive coupling. A method of reducing errors due to capacitive coupling is to employ shielded cables. The additional use of a pair of wires that are entwined is effective in reducing electromagnetic coupling. Surge protection: When an AWS could be subject to unintentional high-voltage inputs, the installation of a protection mechanism is indispensable to avoid possible destruction of the equipment. High-voltage input can be induced from magnetic fields, static electricity and, especially, from lightning. Digital isolation: Electrical modules are used to acquire digital input signals while breaking the galvanic connection between the signal source and the measuring equipment. The modules not only isolate, but also convert the inputs into standard voltage levels that can be read by the equipment. 1. The design and capability of an AWS depend critically upon the method used to power it. The most important characteristics of an AWS power supply are high stability and interference-free