The Dart3 has 2 x different digital output options available. These are:
- Switched Ground Output
- Most common and widely used for most applications
- Pin 12 - Yellow Wire
- Switched Power Out
- Can be used for some specialist applications
- Pin 3 - Purple Wire
Switched Ground Outputs
Switched ground outputs provide a path to ground when 'on' (i.e. allow a path for current, closing the circuit) and are high impedance when off (breaking the circuit). This is an example of a low side switch.
The function of the digital output can be set in system parameters. Functions include:
- 3.5-5V output voltage
- Current limited to 500mA
- Pin 3 - Purple Wire
Switched Power Out
The Dart3 has a switched power rail. This is generally used to provide power to a DM RFID reader.
When this output is ON, 3.5-5V is supplied. So this could be connected to the positive terminal of such a peripheral.
Parameters are the same as for Switched Ground outputs. Set the function to determine what causes the output to turn on or off.
Remote Switching via the Server
Digital Outputs can be used for remote switching. i.e. to turn a pump or lighting tower off or on.
One example is how a vehicle can be immobilised by sending a command from the server - which is covered here Driver ID & Immobilisation methods with Powered Devices. Telematics Guru has an 'immobilise' button, which causes a message to be sent down to the device to turn any outputs set up with function "Immobiliser" on or off. This same message can be sent from a 3rd party server using the OEM WebAPI - contact DM support for documentation.
Any outputs set up with function "Digital Output" can similarly be controlled by sending a different message to control outputs.
Remote Switching Latency
It is important to note that all of our devices (even hard-wired vehicle trackers) will sleep during periods of inactivity. For example, the Dart3 default tracking parameters are for the device to heartbeat hourly when the ignition is off and the asset is stationary. Between heartbeats, the device will sleep and the cellular modem will be off.
This means that a command sent down to the device will not be received until the next heartbeat, when the device next connects.
If we require low latency output switching, to improve upon this, we can set this parameter under the upload settings, available on hard-wired devices.
This will cause the device to attempt to remain connected to the network all the time. In practice, generally the network will kill the connection after 5-10 minutes. So the device additionally needs to be set to upload often enough to keep the connection alive. This can be done by setting the heartbeat interval to a sufficiently short window (may require some experimenting)
This will allow commands to toggle outputs to be received by the device within 60 seconds in most cases. It should be noted that this functionality was not designed with live control applications in mind.