can327: ELM327 driver for Linux SocketCAN

Authors

Max Staudt <max@enpas.org>

Motivation

This driver aims to lower the initial cost for hackers interested in working with CAN buses.

CAN adapters are expensive, few, and far between. ELM327 interfaces are cheap and plentiful. Let’s use ELM327s as CAN adapters.

Introduction

This driver is an effort to turn abundant ELM327 based OBD interfaces into full fledged (as far as possible) CAN interfaces.

Since the ELM327 was never meant to be a stand alone CAN controller, the driver has to switch between its modes as quickly as possible in order to fake full-duplex operation.

As such, can327 is a best effort driver. However, this is more than enough to implement simple request-response protocols (such as OBD II), and to monitor broadcast messages on a bus (such as in a vehicle).

Most ELM327s come as nondescript serial devices, attached via USB or Bluetooth. The driver cannot recognize them by itself, and as such it is up to the user to attach it in form of a TTY line discipline (similar to PPP, SLIP, slcan, ...).

This driver is meant for ELM327 versions 1.4b and up, see below for known limitations in older controllers and clones.

Data sheet

The official data sheets can be found at ELM electronics’ home page:

How to attach the line discipline

Every ELM327 chip is factory programmed to operate at a serial setting of 38400 baud/s, 8 data bits, no parity, 1 stopbit.

If you have kept this default configuration, the line discipline can be attached on a command prompt as follows:

sudo ldattach \
       --debug \
       --speed 38400 \
       --eightbits \
       --noparity \
       --onestopbit \
       --iflag -ICRNL,INLCR,-IXOFF \
       30 \
       /dev/ttyUSB0

To change the ELM327’s serial settings, please refer to its data sheet. This needs to be done before attaching the line discipline.

Once the ldisc is attached, the CAN interface starts out unconfigured. Set the speed before starting it:

# The interface needs to be down to change parameters
sudo ip link set can0 down
sudo ip link set can0 type can bitrate 500000
sudo ip link set can0 up

500000 bit/s is a common rate for OBD-II diagnostics. If you’re connecting straight to a car’s OBD port, this is the speed that most cars (but not all!) expect.

After this, you can set out as usual with candump, cansniffer, etc.

How to check the controller version

Use a terminal program to attach to the controller.

After issuing the “AT WS” command, the controller will respond with its version:

>AT WS


ELM327 v1.4b

>

Note that clones may claim to be any version they like. It is not indicative of their actual feature set.

Communication example

This is a short and incomplete introduction on how to talk to an ELM327. It is here to guide understanding of the controller’s and the driver’s limitation (listed below) as well as manual testing.

The ELM327 has two modes:

  • Command mode

  • Reception mode

In command mode, it expects one command per line, terminated by CR. By default, the prompt is a “>”, after which a command can be entered:

>ATE1
OK
>

The init script in the driver switches off several configuration options that are only meaningful in the original OBD scenario the chip is meant for, and are actually a hindrance for can327.

When a command is not recognized, such as by an older version of the ELM327, a question mark is printed as a response instead of OK:

>ATUNKNOWN
?
>

At present, can327 does not evaluate this response. See the section below on known limitations for details.

When a CAN frame is to be sent, the target address is configured, after which the frame is sent as a command that consists of the data’s hex dump:

>ATSH123
OK
>DEADBEEF12345678
OK
>

The above interaction sends the SFF frame “DE AD BE EF 12 34 56 78” with (11 bit) CAN ID 0x123. For this to function, the controller must be configured for SFF sending mode (using “AT PB”, see code or datasheet).

Once a frame has been sent and wait-for-reply mode is on (ATR1, configured on listen-only=off), or when the reply timeout expires and the driver sets the controller into monitoring mode (ATMA), the ELM327 will send one line for each received CAN frame, consisting of CAN ID, DLC, and data:

123 8 DEADBEEF12345678

For EFF (29 bit) CAN frames, the address format is slightly different, which can327 uses to tell the two apart:

12 34 56 78 8 DEADBEEF12345678

The ELM327 will receive both SFF and EFF frames - the current CAN config (ATPB) does not matter.

If the ELM327’s internal UART sending buffer runs full, it will abort the monitoring mode, print “BUFFER FULL” and drop back into command mode. Note that in this case, unlike with other error messages, the error message may appear on the same line as the last (usually incomplete) data frame:

12 34 56 78 8 DEADBEEF123 BUFFER FULL

Known limitations of the controller

  • Clone devices (“v1.5” and others)

    Sending RTR frames is not supported and will be dropped silently.

    Receiving RTR with DLC 8 will appear to be a regular frame with the last received frame’s DLC and payload.

    “AT CSM” (CAN Silent Monitoring, i.e. don’t send CAN ACKs) is not supported, and is hard coded to ON. Thus, frames are not ACKed while listening: “AT MA” (Monitor All) will always be “silent”. However, immediately after sending a frame, the ELM327 will be in “receive reply” mode, in which it does ACK any received frames. Once the bus goes silent, or an error occurs (such as BUFFER FULL), or the receive reply timeout runs out, the ELM327 will end reply reception mode on its own and can327 will fall back to “AT MA” in order to keep monitoring the bus.

    Other limitations may apply, depending on the clone and the quality of its firmware.

  • All versions

    No full duplex operation is supported. The driver will switch between input/output mode as quickly as possible.

    The length of outgoing RTR frames cannot be set. In fact, some clones (tested with one identifying as “v1.5”) are unable to send RTR frames at all.

    We don’t have a way to get real-time notifications on CAN errors. While there is a command (AT CS) to retrieve some basic stats, we don’t poll it as it would force us to interrupt reception mode.

  • Versions prior to 1.4b

    These versions do not send CAN ACKs when in monitoring mode (AT MA). However, they do send ACKs while waiting for a reply immediately after sending a frame. The driver maximizes this time to make the controller as useful as possible.

    Starting with version 1.4b, the ELM327 supports the “AT CSM” command, and the “listen-only” CAN option will take effect.

  • Versions prior to 1.4

    These chips do not support the “AT PB” command, and thus cannot change bitrate or SFF/EFF mode on-the-fly. This will have to be programmed by the user before attaching the line discipline. See the data sheet for details.

  • Versions prior to 1.3

    These chips cannot be used at all with can327. They do not support the “AT D1” command, which is necessary to avoid parsing conflicts on incoming data, as well as distinction of RTR frame lengths.

    Specifically, this allows for easy distinction of SFF and EFF frames, and to check whether frames are complete. While it is possible to deduce the type and length from the length of the line the ELM327 sends us, this method fails when the ELM327’s UART output buffer overruns. It may abort sending in the middle of the line, which will then be mistaken for something else.

Known limitations of the driver

  • No 8/7 timing.

    ELM327 can only set CAN bitrates that are of the form 500000/n, where n is an integer divisor. However there is an exception: With a separate flag, it may set the speed to be 8/7 of the speed indicated by the divisor. This mode is not currently implemented.

  • No evaluation of command responses.

    The ELM327 will reply with OK when a command is understood, and with ? when it is not. The driver does not currently check this, and simply assumes that the chip understands every command. The driver is built such that functionality degrades gracefully nevertheless. See the section on known limitations of the controller.

  • No use of hardware CAN ID filtering

    An ELM327’s UART sending buffer will easily overflow on heavy CAN bus load, resulting in the “BUFFER FULL” message. Using the hardware filters available through “AT CF xxx” and “AT CM xxx” would be helpful here, however SocketCAN does not currently provide a facility to make use of such hardware features.

Rationale behind the chosen configuration

AT E1

Echo on

We need this to be able to get a prompt reliably.

AT S1

Spaces on

We need this to distinguish 11/29 bit CAN addresses received.

Note: We can usually do this using the line length (odd/even), but this fails if the line is not transmitted fully to the host (BUFFER FULL).

AT D1

DLC on

We need this to tell the “length” of RTR frames.

A note on CAN bus termination

Your adapter may have resistors soldered in which are meant to terminate the bus. This is correct when it is plugged into a OBD-II socket, but not helpful when trying to tap into the middle of an existing CAN bus.

If communications don’t work with the adapter connected, check for the termination resistors on its PCB and try removing them.