Data Acquisition

Command errors are reported immediately after a command has been sent to the standalonedevice. They indicate that all or part of the command was either not understoodbecause of incorrect transmission from the host PC, the command contained an incorrectsyntax or alternatively the command itself was not actionable. If a command refers to aset of channels, some of which are not used or are an incorrect type, then the appropriateerror response is generated but the command is executed for those channels to which itapplies.Channel errors are reported when an error occurs in the measurement from a particularchannel. The method of reporting channel errors often varies, depending on the standalonedevice. As well as reporting an error, some devices return a default data error value(i.e. 99999.9) in the logged or displayed data. An example of this type of error is when achannel is read and the analog signal on that channel is out of range.Operational errors occur when an operational limitation prevents the correct execution ofa command. For example, an operational error occurs when the command input bufferbecomes full. This can be due to the command being too long or successive commandsbeing sent too quickly. An operational error would also occur when the data memorybecame full.The error message is typically sent in some shorthand ASCII notation, but can be sent in amore descriptive (verbose) ASCII form. In all cases the error format is determined by theconfiguration in which the remote device was requested to send it. The format is usuallyset by a command switch. It should be noted that error messages can be turned off –typically by a command switch.System commands are used to perform system initialization, hardware initialization,variable initialization, system parameters setting (such as the time, date and password), orreturn the system status.The versatility and simplicity of use of stand-alone devices lies in the fact that manydifferent types of sensor can be interfaced to the input channels of the device. In mostcases it does not matter to which channel a sensor is connected, provided the device isinformed which sensor is connected to each channel. The rate at which each channel issampled is entirely variable. Some channels can be configured to be sampled continuously,while others take a measurement only when certain operating conditions aremet. Once instructed to perform a data acquisition or control task (or many complicatedtasks), the remote device will continue to operate by itself, taking measurements, storingdata if required, or sending data back to the host PC.