Data Acquisition

In many applications, the stand-alone logger/controllers are not contained within thesame factory or industrial plant, but located at a distance beyond the capabilities of radiomodem communications. An example of this would be a remote electricity sub-stationused to monitor alarm conditions, provide on-line voltage, current, and power readings toa central control room. Communications between the host PC and the remote units via thetelephone network is shown in Figure 7.6. A dedicated phone line allows frequent uploadingof data to the host PC, constant monitoring of alarms and on-line system control,where required.Stand-alonelogger / controllerThermocouples Strain gauges RelaysModemStand-alonelogger / controllerModemTelephone lineThermocouples Strain gauges RelaysStand-alonelogger / controllerRS-232Thermocouples Strain gauges RelaysHost ComputerStand-alonelogger / controllerThermocouples Strain gauges RelaysFigure 7.6A remote connection to a logger/controller network via the telephone network7.3 Stand-alone logger/controller hardwareThe important features that give stand-alone logger/controllers the power and flexibilityto operate, either as stand-alone devices, or as part of a distributed network,fundamentally lie in their relatively complex hardware structure. The simplified hardwareschematic of a typical stand-alone logger/controller is shown in Figure 7.7.Figure 7.7Simplified hardware schematic of a stand-alone logger/controller