Data Acquisition

Other off-the-shelf software packages that can be used with remote stand-alone devicesare:• Any terminal emulator/communications package• Spreadsheet packages such as Excel, Lotus 123, Framework, QuattroIn all cases, the format of the commands received and the data sent in return by theremote device is still in simple ASCII format, dictated by the firmware in the remotedevice.7.8 Considerations in using stand-alone logger/controllersData acquisition and control using stand-alone logger/controllers is an orderly process.When designing a system that utilizes these devices, users should consider the following:• The first is the number of sensors and control outputs required, and theirlocation in relation to the host PC. This determines how manylogger/controllers are required and their location. Where the analog input ordigital I/O requirements are greater than can be provided by a single unit, butthe increased channel requirements are localized, expansion modules can beused. If the increased I/O capability must be distributed, more than onelogger/controller has to be used and can be connected as part of a distributednetwork. Where the units are remotely located (e.g. at a remote weatherstation or an electricity sub-station located in the country) and it is notpractical for these devices to be connected directly to the host PC, then twooptions are available. These are: an operator uploads data using a memorycard, and the remote device is connected via a modem to the host PC.• The volume of data that is to be logged. Stand-alone logger/controllers onlyhave a limited amount of memory. How quickly the memory becomes fulldepends on the number of input channels to be read and their scan rate. If onlya limited amount of data is to be stored then the internal memory may besufficient. Increased capacity can be provided by a memory card. If theinternal memory or the memory card becomes full frequently, then thelogger/controller will require connection, either directly or via modems.• How often stored data must be uploaded to the host PC. Equally important asmemory considerations, upload of data to a host PC, is largely determined bythe type of application. Logging applications in which the data are gatheredfor analysis over an extended period (e.g. weather monitoring applications),clearly do not require constant uploading. The frequency of uploading wouldonly depend on the memory capacity of the device and the amount of databeing logged. Critical applications requiring constant uploading of processdata, feedback of alarms and on-line control would need to be directlyconnected to the host PC.• The availability of power. Where mains power is not available, the standalonelogger/controller is powered from a battery supply. The battery life isdetermined by a number of factors and is not unlimited. In time, the batterywill need to be charged or replaced.