Data Acquisition

How frequently logged data is uploaded depends firstly on how critical the immediateanalysis of data is to the system or process being controlled, and secondly on how muchmemory is available and how quickly it will become full.How quickly the memory will become full is important for two reasons. During failureof the host PC or communications interface, there must be enough memory to allow datalogging to continue without loss of data. In addition, a device connected to the host PCvia a multidrop network can only return data when requested by the host PC. Where alarge number of units are connected to the host, the memory of each unit must be largeenough to allow data logging to continue without loss of data, until the next time the hostrequests a data upload.Aside from this specific limitation, it is good practice to recover data as often aspossible since any sensor errors, power supply failures or problems with the unit itselfwill be detected early, thereby increasing system reliability. In addition, frequent datarecovery will help to minimize the chance that data may be lost due to device failuressuch as battery-backed memory failure.Another useful configuration is the connection of remote logger/controllers to the host PCusing modems via either a telephone network or radio communications. In large factoriesor industrial plants, where one or more devices are distributed over a wide physical area,the closest logger/controller to the host PC may be too far away or too greatly affected bynoise to allow connection to a host PC via the RS-232 communications interface. In suchapplications, the use of radio communications is a practical solution. When radiocommunications take place between the host PC and the distributed network, allcommunications must go through the logger/controller to which the remote radio modemis connected. This is shown in Figure 7.5.Stand-alonelogger / controllerThermocouples Strain gauges RelaysStand-alonelogger / controllerThermocouples Strain gauges RelaysHost ComputerStand-alonelogger / controllerRS-232Radio communications linkThermocouples Strain gauges RelaysModemModemStand-alonelogger / controllerThermocouples Strain gauges RelaysFigure 7.5Remote connections to a logger/controller network via radio communications