Data Acquisition

When using this method, the stand-alone device is programmed to take data readings oneach channel at a specified scan interval. The format of the scan interval can be every xseconds, minutes, hours or days. Where no interval is defined, scanning will occur asrapidly as possible.When using this method the occurrence of an event on a pre-determined digital input orcounter input is used to trigger the scanning of all channels in the schedule.The trigger events commonly used are:• Trigger on positive voltage transitions (low to high) of a digital input• Trigger on negative voltage transitions (high to low) of a digital input• Trigger on both positive and negative transitions of a digital input• Trigger after ‘n’ counts of a counter inputWhen schedule scanning occurs subject to a digital event, the stand-alone device mustbe constantly checking the required digital inputs or counter inputs for the event to occur.This requires that the device must not go into a low power sleep mode. As there arelimitations on the speed at which the device can do a check scan of the required digital orcount inputs, care must be taken to ensure that the speed of the trigger event is not fasterthan the check scan rate, otherwise events may be missed.In addition to the various methods of time interval triggering or event triggering aschedule channel scan, it is often possible to enable/disable the trigger event using thestate of one or more other digital inputs. This would be very useful, for example, if analarm limit was reached and this condition was used to enable the trigger event thatactivated the channel scans.Five types of schedule are typically defined:• Immediate schedules• Report schedules• Polled schedule• Statistical schedules• Alarm schedulesImmediate schedules are used for inspecting and testing input channels and sensors.When this schedule is triggered, either by a command from the host PC or an alarmcondition becoming true, the designated channels are scanned only once and data isreturned to the host only. The execution of an immediate schedule channel scan does notdisrupt any report schedules that may be in progress and have a higher designatedpriority.This type of schedule is used for repeated data acquisition from selected input channelsand forms the program building blocks of the stand-alone logger/controller program.