Data Acquisition

Expansion modules provide increased localized channel capacity for a data acquisitionsystem using stand-alone logger/controllers. Expansion connectors provide an extensionof the internal data and control bus lines of the logger/controller. When connected,additional analog input channels, digital I/O channels, and counter channels, on theexpansion module, are treated as if they were part of the logger/controller to which theyare attached. This is shown in Figure 7.12.PowerSupplyHost ComputerStand-alonelogger / controllerExpansionModuleExpansionModuleThermocouples Strain gauges RelaysThermocouples Strain gauges RelaysThermocouples Strain gauges RelaysFigure 7.12Connection of expansion modules7.4 Communications hardware interfaceCommunications interface standards define the electrical and mechanical details thatallow communication equipment from different manufacturers to be connected togetherand to function efficiently. Two standards are commonly employed for communicationsbetween PCs and stand-alone or distributed logger/controllers:• RS-232 standard• RS-485 standardAs the RS-232 communications interface is standard on most IBM PCs and compatibles(i.e. COM1 and COM2 ports), stand-alone devices first used this interface for communicationto the PC. The RS-232 interface is discussed in detail in Chapter 6, howeversome of the most basic setup parameters for stand-alone logger/controllers are discussedbelow.Usually, the comms port parameters (i.e. start bits, data bits, stop bits) are fixed. The onlyuser setable parameter is the baud rate, which is typically set by dip switches on thedevice. Commonly used baud rates are 300, 1200, 2400, 4800, 9600 and 19200. Theoptimum baud rate setting is a compromise between the speed necessary to transmit thenecessary amount of data over the communications interface, and the speed required sothat data can be transferred without error over the distance that the host PC is locatedfrom the local stand-alone logger/controller.Stand-alone loggers/controllers are typically equipped with a DB-9 connector. The pinallocations commonly used with the DB-25 and DB-9 connectors for the RS-232Cinterface are not quite the same and often provide a trap to the beginner.