I/O and Interfaces

The majority of classic AVL devices have more than two wires that normally required for power supply. Their connectors might contain up to 20 pins and even more, though average amount is something between 6 and 10. What are they used for?

The answer is pretty obvious: you can connect various sensors, buttons and extra equipment which allow to acquire the telemetry data or even control vehicle mechanisms remotely. The most common examples are ACC state, emergency button, door sensor, fuel level sensor, mic and speaker, camera, etc. Let’s take a closer look at the types of inputs, outputs and other interfaces used in vehicle GPS trackers.

Discrete inputs

Discrete inputs (sometimes they are also called as digital inputs which is not quite correct) are the simplest type of inputs, which you can find even in cheap GPS trackers. They called this way because they can detect just two statuses: ON and OFF. With the discrete inputs you can obtain just basic information about the state of connected equipment, to see if it is enabled or disabled.

Here are some common examples:

  • ACC / Ignition status
  • Door opening sensors
  • Emergency button
  • Car alarm triggering

Discrete input can be positive or negative. If the input is positive, it detects the “high” state (i.e. voltage level between GND and INPUT is higher than +5V), if it’s negative it allows to fix the “low” state (i.e. voltage level between GND and INPUT is less than +1V, which usually means that INPUT is connected to GND).

Analog inputs

Analog inputs differ from discrete ones. In the analog signal the voltage level is used to represent the information – usually about the changes in physical phenomena, such as temperature, pressure, humidity or position of hydrometric float. In other words with analog inputs you can obtain more information from the connected equipment.

In fleet tracking analog inputs are often used for connecting:

  • Fuel level sensor
  • Temperature sensor
  • Humidity sensor
  • Tyre pressure sensor

However, analog signals are affected with noises, especially if vehicle electrical system is not in a good condition or cable is too long. It causes inaccuracies in the measurements. To acquire more precise data it is recommended to use frequency inputs or, better, serial interfaces.

Frequency inputs

Frequency inputs is the example of evolution of the analog inputs. The signal is formed not with an amplitude, but with frequency modulation, which makes it less affected by noises. It is similar to AM and FM radio. Though frequency inputs are rarely used today, you can still find fuel sensors which use this type of modulation.

Impulse inputs

Impulse input represents the amount of impulses within a certain period of time. Impulse inputs are widely used for connecting fuel flow meters. By calculating the amount of impulses the system can measure the amount of fuel which passed through the fuel line.

Serial interfaces

With serial interfaces you can connect smart sensors and other digital devices (i.e. camera). It’s the most reliable and modern way to transfer data within the range of a few meters. That’s exactly what is needed for connecting GPS tracking device with various sensors and extra hardware:

  • Digital fuel level sensors
  • Driver identification systems
  • CAN bus adaptors
  • Digital cameras

The most common standards for serial interfaces are RS232 and RS485. While the first allows connecting only one device, with the second (and the newer) one you can connect as many as 32 devices using the same pairs of wires. The possibility of connecting multiple fuel level sensors is required when you have two or more tanks in the vehicle. Another example – using several fuel level sensor in one large fuel tank for better accuracy.


1-Wire is a new and modern technology, that features only one wire to connect multiple devices. 1-Wire technology delivers highly accurate readings at a low cost. In fleet tracking applications 1-Wire is used to connect one or many temperature sensors and driver identification kits (iButton).


With outputs you can control various kind of external equipment, for example:

  • Engine cut relay (immobilizer)
  • Engine pre-starting heater
  • Engine starting device
  • Siren, etc.

Outputs can have two states: OFF and ON. You can control them remotely by sending SMS or GPRS commands. In some vehicle trackers there is the possibility to program a primitive on-board logic, for example: “If input #1 is ON and input #2 is OFF, then enable output #1 for 3 seconds”.

What’s important to remember about the outputs in vehicle trackers:

It’s better if outputs are controlled over GPRS, while SMS channel remains as an option. The reason is GPRS/IP provides more interactive command feedback and, thus, better user experience. With GPRS you don’t need to spend money on SMS each time you starting the engine remotely, then wait for 15-30 seconds for device’s response (plus another SMS). Instead you just tap the button in mobile app or web interface and it responses back within a couple of seconds, and no extra fees for SMS delivery required.

Max current for outputs is usually not very high, something like 100-200 mA. If you need to connect something more power consuming, you should use external relay. However, some GPS trackers (e.g. Queclink GMT100) already have built-in relay which can stand for larger currents.