Data Acquisition

Random access memory (RAM) is generally used in stand-alone systems for the storageof measurement data, control, and system parameters. The two most popular types ofRAM technology are static and dynamic. Dynamic RAM requires periodic updating orrefreshing, whereas static RAM does not require refreshing. However, the advantage ofdynamic RAM over static RAM is a far greater memory capacity for a given area ofsilicon.Dynamic RAM is suitable for a personal computer used in an office environment wherememory capacity is an important requirement. However, in a stand-alone system theadvantage of static RAM lies in its ability to maintain the data contents, using a backupbattery, in the absence of main power. This can be achieved with relative ease, becausestatic RAM does not require refreshing, even in standby mode.Electrically erasable programmable read only memory (EEPROM) is a non-volatilememory technology, generally used for the storage of limited configuration data andcontrol parameters. The moderate memory capacities and slow write cycle of EEPROM(typically 10 milliseconds) limit its application.Flash programmable read only memory (FlashPROM) is also a non-volatile memorytechnology, and is used for both mass data and program storage. FlashPROM is availablein memory capacities ranging from 32 Kbytes to 2 Mbytes. The much shorter write cycleof FlashPROM is achieved at the expense of having to erase data on the chip in fixed-sizeblocks rather than a byte at a time.Similar to RAM, plug-in memory cards are also used in stand-alone systems for thestorage of measurement and control data. Although there are a number of memory cardmanufacturers, the Personal Computer Memory Card International Association(PCMCIA) standard cards, have become very popular for use with notebook computers,particularly PCMCIA modems.PCMCIA memory cards are available in ROM, one-time programmable ROM, staticRAM, UV EPROM, flashPROM and EEPROM technologies. The static RAM PCMCIAcards are the obvious choice for data storage in stand-alone systems, and are currentlyavailable in memory capacities ranging from 64 Kbytes to in excess of 8 Mbytes.An important advantage of memory cards in a stand-alone system is their ability to beremoved and replaced with another blank card in the field, providing a convenient datatransfer mechanism. Additionally, memory cards allow the user to purchase and installonly the memory capacity required for a particular application.The real time clock (RTC) is an important part of any stand-alone system. It not onlyprovides the necessary date and time information, but also provides periodic and alarmfunctions for triggering the reading of sensors and controlling outputs under programcontrol.The RTC will be connected to the associated power management circuitry, allowing thesystem to remain in standby mode conserving power, until the RTC periodic event oralarm event wakes the system up. The control software is then able to read and recordsensor data and manipulate control outputs, before returning the system to the low powerstandby mode (sleep mode).