In the simplest terms, a channel on a DT80 data logger is an input or output.The logger has:
- external channels - the terminals on the outside of the logger to which you connect sensors and transducers.
They can be an analog, digital, and serial sensors.
- internal channels - internal sources of information, such as time and date from the logger's clock, or variables used in calculations.
These channels are normally scanned by the DT80 data logger on a regular time interval, by grouping the channels within a schedule. The scan interval can be varied by changing the schedule rate. There can also be more than one schedule in a logger.
The following example measures a number of analog channels, such as a type K thermocouple, a voltage from a voltage sensor, a current input from a current device, as well as the digital state of a digital sensor, every 5 seconds.The dataTaker data logger will follow the flow chart below:
There are many variations to channels and schedules, often utilised for more complex tasks. This is described further in the document.
A “channel” defines each task within a schedule. If the task is a measurement it normally results in a single data value. Another common name for a channel is a “data point” as it returns a single data value.
There are three main categories of channels on dataTaker data loggers: analog channels, digital channels, and serial channels. These categories define the signal types that may be connected to the terminals associated with the channel. Each main category has its own channel numbering such that there is an analog input channel number one, which is different and physically separate from digital channel one.
For example, the channel specification “1V” defines a channel that measures voltage on the terminals for analog channel one, whereas the channel specification “1DS” defines a channel that measures the digital logic state on the terminal for digital channel one.
Each analog input channel on a DT80 is a 4-wire connection (see Figure 2 Analog Channel Terminal Layout) that allows voltage, current, resistance and frequency to be measured. These are the fundamental signals output by most sensors. It is not necessary to use all four terminals on each channel— two are often adequate. Most DT8x models have more than one analog channel.
Figure 2 - Analog Channel Terminal Layout
The exact function of each terminal varies depending on how the channel is programmed. In general terms:
- The * ("Excite") terminal can be a voltage input (relative to # terminal), or it can provide sensor excitation (for example, for resistance measurement) See Sensor Excitation in the User Manual
- The + ("Plus") terminal is a voltage input (relative to – or # terminal)
- The – ("Minus") terminal is a voltage input (relative to # terminal)
- The # ("Return") terminal is normally used as a common or return terminal. It can also be used as a current input, using the DT80's internal shunt resistor.
Between the connection terminals of a channel and the analog to digital converter there is a multiplexer (which selects the channel) and a programmable instrumentation amplifier (which sets the gain). The multiplexer is essentially a switch that directs signals from the channel terminals to the amplifier inputs. Many different connections are possible.
An analog channel can read signals from sensors, in differential or single ended mode, and provide excitation to sensors.
A digital channel can be a digital state input, pulse count, or a phase encoder position. A digital channel is a 1-wire connection (plus shared ground) that allows digital states to be measured and digital pulses to be counted.
The DT80 supports two main classes of "smart sensor":
- SDI-12 (Serial Data Interface – 1200 baud) based sensor networks
- other serial sensor devices with an RS232/422/485 interface, such as weighing machines, barcode scanners
A dataTaker channel is very versatile, but it does not automatically know what type of sensor is connected - you must tell it by using commands.
A channel is defined by a "Channel Type" that determines how the multiplexer is switched and how the readings are to be processed. The channel type also implies the main category, analog or digital, and hence defines whether the analog or digital channel terminals are used at the input connections for the channel.
There are more than thirty different channel types. (For example: voltage, current, resistance, frequency, thermocouple, RTD, LM35, bridge...)
The same channel may be read using different channel types. For example a thermocouple may be read as a thermocouple or a voltage. The command string
A dataTaker channels default behaviour can be modified by the use of channel options. Some modifications include supplying different excitation to the sensor, statistical reporting and varying the channel output format. Each dataTaker channel can have multiple channel options.
A “schedule” tells the data logger when to perform a group of tasks defined by the channels within that schedule. It specifies
- which channels to scan (a schedule can contain one, two or many channels)
- when to scan the specified channels — for example, whenever 5 seconds have elapsed (repeating every 5 seconds), whenever a door closes (scan on digital event), or whenever an alarm occurs.
The DT80 data logger has eleven basic schedules (labelled A, B, C,…K). All channels defined within a schedule are scanned at the same time. Many channels can be defined within each schedule, allowing you to group channels into schedules appropriate to your task.
The schedule has three main parts:
- schedule identification
- schedule trigger
- list of channels to scan
For more information on schedules, please refer to the Schedules section of the DT8x User's Manual.
There are three distinct types of schedules that are identified by their schedule identifier, also known as the schedule ID.
The three types of schedules are:
- Triggered Schedules
- Polled Schedules
- Statistical Sub-Schedule
Triggered schedules are triggered by a time event, digital input event, counter input event or combinations of these variables. They scan their channel lists at intervals and times determined by the trigger.
Triggered schedules are identified using the capital letters. The DT80 data logger has eleven triggered schedules labelled A,B,C…K.
The polled schedule scans its listed channels only when the host computer or an ALARM issues the X or Xn command.The DT80 data logger allows all triggered schedules (A through K) to also be polled with the Xn command. The polled schedules are not supported by the driver with the intermediate level VIs.
The statistical sub-schedule defines when to scan channels that include statistical summary values. Examples of statistical summary values include average, minimum and maximum values over a period of time. The statistical sub-schedule is identified as schedule S. To generate statistical summary values the channels must be scanned at a faster rate and then the statistical summary values are calculated and reported back at a slower rate. The statistical sub-schedule defines the faster rate.
There is only one statistical sub-schedule so all channels that include statistical values are scanned at the same time. However, they may report statistical summary values at different rates. The statistical sub-schedule can be triggered in the same ways as triggered schedules (for example, time or digital event).
The statistical sub-schedule is different to all other schedule types as it does not have a channel list directly associated with it. Channels are indirectly associated with the statistical schedule by including the relevant statistical channel option in the channel option definition for the channel. The use of the option causes the channel to be scanned every time the statistical sub-schedule is triggered. The statistical summary value is reported whenever the triggered schedule that the channel is associated with is triggered. Thus for meaningful summary data, the statistical sub-schedule should be defined to scan faster than the fastest triggered schedule that includes statistical summary data.
A scan trigger defines when a schedule will scan its associated channels. The trigger can be a time interval, or a digital event. The trigger can also be enabled or disabled by the state of one or more digital inputs, using the trigger while feature.
The time interval of the trigger can be in days, hours, minutes, or seconds. The DT80 data logger also supports thousandths of seconds as a valid time trigger interval. Thousandths of a second is the same unit as milliseconds (ms).
NOTE: The fastest sample rate of the DT80 data logger is approximately every 250 ms with multiple channels. The more analog channels, the slower the sampling rate on each channel. This is due to the settling time of the relays.
The digital event trigger can be on a transition of a digital channel, after n counts of a low speed counter, or after any counts on a high speed counter.
The channel list is a set of channels that will be scanned whenever the schedule they are associated with is triggered.
To define a channel in the driver using the high level and intermediate level VIs, you use the channel list control.
The channel list control is an array which contains a cluster called "intermediate channel config" which describes a channel on a dataTaker data logger.
The following image is the channel list control used within the driver
The intermediate channel config cluster describes a channel on a dataTaker data logger. There are four main components within the cluster.
They are the channel number, channel type, channel wiring and the channel option list.
The channel number is the number of the channel that the sensor is wired to.
Note: If you are familiar with the dataTaker syntax, you can enter the entire channel definition in this field, as long as the channel type is listed as "none". An example of this is :
The channel type is the type of sensor or signal that is being read by the dataTaker channel. For example V specifies a voltage input. If the selection is "none" this is not used, and the dataTaker channel is assumed to be correctly defined in the channel number field.
For more information, please refer to the Channel Types section of the DT8x User's Manual.
The channel wiring field refers to how the sensor is wired to the dataTaker channel. Default is most commonly differential for analog input channels, and for digital inputs, it would be a switch between the digital channel and ground.
For more information, please refer to the Sensors & Channels section of the DT8x User's Manual.
The channel option list is an array of strings, where each element of the array can be a single channel option, or a list of options separated by commas. Please note that brackets are not required to be entered in this field, as the VIs enclose the channel options in brackets within the code.
For more information, please refer to the Channels, Channel Options section of the DT8x User's Manual.
To define a schedule in the driver using the high level and intermediate level VIs, you use the schedule config list control.
The schedule config list control is an array which contains a cluster called "schedule config" which describes a schedule on a dataTaker data logger.
The following image is the schedule config list control used within the driver.
The schedule config cluster describes a schedule on a dataTaker data logger. There are four main components within the cluster.
They are the Schedule ID, trigger type, trigger time, and channel list.
The schedule ID is the schedule identifier of the schedule. Only the triggered schedules and the alarm schedule is listed.
The trigger type is the type of time interval used to trigger the schedule.
The DT80 can sample multiple channels at approximately 4 tp 5Hz. Normal sample mode allows for maximum mains frequency noise rejection. You can adjust settings on the DT80 to sample at faster rates. For example, closing the relays when sampling with only one channel, and adjusting the mains frequency parameter can enable the DT80 to sample up to 25Hz.
For more information, please refer to the Schedules section of the DT8x User's Manual.
The trigger time is the actual time interval used for the trigger schedule. For example, if trigger type is in "thousandths of a second", and trigger time is 500, the time interval will be 500 "thousandths of a second", or 0.5 of a second. Thousandths of a second is the same unit as milliseconds (ms).
The channel list is an array of the channel list cluster. The channel list is described above.