Diagnostic interfaces

Presentation of ELM interfaces
Where can I buy an ELM327 interface?
Instructions for ELM327 interfaces
Operating principle of the ELM327 interface
Tips and advice for using ELM327 interfaces
ELM circuit for electronics technicians
Other types of interfaces

Presentation of ELM interfaces

There are several types of interface. The most common use a chip made by ELM Electronics. The ELM327 is the most widely used and practical, supports all OBD protocols, i.e. KWP, PWM, VPN and CAN. The other circuits only support one protocol: The ELM320 only supports PWM; the ELM322 supports VPN and the ELM323 KWP.

Which communication for my interface?

To our knowlegde, there are 4 types of ELM327 interface:

Below for illustration, is a photo of the 4 types of interface using this integrated circuit (electronic chip):

4 types of ELM327

From left to right: a metal box USB interface, a plastic box, a Bluetooth version and the latest WiFi version

Compatibility PC Windows software Mac OS X software Android Application iOS Application
ELM327 RS232
ELM327 Bluetooth
ELM327 WiFi

Despite appearances, these 4 interfaces are practically identical, at least electronically. Only their external aspect and the connection type differ. At their heart is an ELM327 circuit.

Another thing which may be confusing is the "firmware" version. that may vary. The official versions currently in circulation are 1.3a and 1.4b. Only very advanced users can tell the difference between a 1.3 and a 1.4 as the basic diagnostic functions are identical. On the net, you will also find version 1.5. We have bought one and can confirm that it doesn't exist! It is only a copy of version 1.2 masquarading as 1.2. Check out our tips and advice section for how to know which ELM version you have.

New: Elm Electronics have just released version 2.0 of their ELM327 circuit.

Where can I buy an ELM interface?

Our official store:

Following requests from users asking where they can buy an OBD2 interface compatible with the ELM327 interface, we decided to open our online boutique for automobile diagnostics. If you decide to purchase from our boutique, you will avail of the following advantages:

Buy your OBD2 pack

plastic box
metal box
OBD2 Bluetooth OBD2 WiFi
Purchase OBD2 plastic
Purchase ELM327 Metal Alu
Purchase OBD2 Bluetooth
Purchase OBD2 WiFi

Foreign sellers:

Most sellers come from these countries: China, Hong Kong, USA. Before buying, we recommend you check the following:

Instructions for the ELM327 interfaces

1. Installing the drivers:

2. First connection to the vehicle:

3. Description of LED indicators:

Tip: On some interfaces, there are no labels on the indicators. In this case, when you power on, the red LED lights first followed briefly by: 1. Tx OBD, 2. Rx OBD, 3. Tx PC, 4. Rx PC.

Operating principle of the ELM327 interface

The electronic unit contains 3 main sub-assemblies to operate the interface:

  1. Electrical voltage adaptors: On-board networks in cars have voltage levels that require specific drivers. For an ELM327, as the device supports several protocols, several drivers are necessary. These protocols can be classified into 3 families, CAN networks, K/L lines and PWM/VPN. For more details about these protocols, see our page communication standard.
  2. The ELM327 chip: The integrated circuit, which gives its name to the whole device, is the brains of the interface. It selects the protocol and converts it into one understood by modern PCs. This small circuit (speed 4Mhz) acts as a bridge between the protocols.
  3. Voltage adaptors for the PC: The electronic chip is not in itself able to communication with a computer; it needs to adapt the electrical levels before sending data streams.

The diagram below shows the main function of an interface.

Overview ELM327 operation

After this brief presentation, the term "interface" should mean more. The role of the ELM327 is to act as an interface between 2 different environments: the car, governed by standards with a high level of reliability and a low price, and the IT world more focussed on modularity.

Tips and advice for using ELM327 interfaces

How do I know which software version it is?

The interfaces do not always have the same level of quality. As we mentioned above, the software version displayed is not always reliable. Some versions of the circuit are on sale that are not even listed by the manufacturer (reminder: ELM Electronics). User returns have taught us that some versions, that do not officially exist, are in circulation. To clarify the situation:

We do not need to go into the details as to why these non-official versions exist. What is important as a user is to be able to qualify your interface. I.e. be able to verify its capacity. To do so, we recommend you do the following:

Compatibility Command Expected reply
version 1.2 and +ATKW0OK
version 1.3 and +ATS0OK
version 1.3a and +ATFEOK
version 1.4 and +ATSSOK
version 1.4a and +ATCSM1OK
version 2.0ATAMT 40OK

This little test will enable you to determine what level of function is installed in your interface. Even if this is only a simplified version of our complete procedure, it is effective and fast. Our software is 100% operational with interfaces from version 1.2, even though it uses some commands from version 1.3, there is no impact on function.

ELM circuit ELM for electronics technicians

If your ELM interface breaks down or if you want to create your own circuit, the diagram below will help you. It shows the circuit of anELM327 RS232 interface. The basic circuit is always the same whatever the connection mode (RS232, USB or Bluetooth). The difference lies mainly in the components connected to pins 17 and 18 of U1 (ELM327 integrated circuit).

For USB interfaces, a VCP (Virtual COM Port) component is used to "convert" an RS link into USB like USB adaptor cables <-> RS232. There are currently two families of components in the ELM327. The manufacturer FTDI with the component FT232 (more details on the FTDI site) and Silicon Labs with the component CP210x (more details on the Silicon Labs website).

Bluetooth interfaces use an RS modem.<->Bluetooth.

WiFi interfaces use an RS modem.<->WiFi allows adhoc wireless communication.

ELM327 Interface diagram ELM327 wiring diagram

Click on the images to enlarge

In the event of malfunction:

ELM Electronics has created a very good and practical application note with the voltage levels and a test procedure. This document can be found at the following address: http://www.elmelectronics.com/AppNotes/AppNote02.pdf.

Other types of interfaces

Only ELM interfaces are supported by our software, but other companies have developed interfaces with functions similar to the ELM circuit.

For example: http://www.ozenelektronik.com and their interfaces myOBDics.

On the market, there are also professional products, such as those used by manufacturers such as Clip, Lexia, etc. They remain expensive however. In addition, their special nature makes them usable only by maker family, unlike the ELM.