-Linux SocketCAN driver for ELM327
+.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
+
+ELM327 driver for Linux SocketCAN
==================================
+Out-of-tree version
+--------------------
+
+This is the non-upstreamed version of the elmcan driver.
+Please see out-of-tree.rst for compilation/usage hints.
+
+
+
Authors
--------
-Max Staudt <elmcan@enpas.org>
+Max Staudt <max-linux@enpas.org>
Motivation
-----------
-CAN adapters are expensive, few, and far between.
-ELM327 interfaces are cheap and plentiful.
-
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.
+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 asa quickly as possible in
-order to approximate full-duplex operation.
+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, elmcan is a best-effort driver. However, this is more than
+As such, elmcan 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).
------------------------------------
Use a terminal program to attach to the controller.
+The default settings are 38400 baud/s, 8 data bits, no parity, 1 stopbit.
After issuing the "``AT WS``" command, the controller will respond with
its version::
>
+Note that clones may claim to be any version they like.
+It is not indicative of their actual feature set.
+
How to attach the line discipline
--noparity \
--onestopbit \
--iflag -ICRNL,INLCR,-IXOFF \
- 26 \
+ 29 \
/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.
+
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``" not supported, thus no ACK-ing frames while listening:
+ "``AT MA``" 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), the ELM327 will end reply
+ reception mode on its own and elmcan will fall back to "``AT MA``"
+ in order to keep monitoring the bus.
+
+
- All versions
- No automatic full duplex operation is supported. The driver will
- switch between input/output mode as quickly as possible.
+ 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
- Versions prior to 1.3
These chips cannot be used at all with elmcan. They do not support
- the "``AT D1``", which is necessary to avoid parsing conflicts on
- incoming data, as well as distinction of RTR frame lengths.
+ 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
helpful here, however SocketCAN does not currently provide a facility
to make use of such hardware features.
-- No BUS-OFF state and automatic restart
-
- We currently reset the ELM327 and generate error frames manually.
- In the future, we may be able to use ``can_bus_off()`` and its siblings.
-
Communication example
ID 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
+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::
-To Do list for future development
-----------------------------------
+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).
-- Handle ``write()`` error
+``AT D1``
+ DLC on
-- Rename ``elm327_panic()``
+ We need this to tell the "length" of RTR frames.
-- No auto-restart in ``elm327_panic()``?
-- Stop current function when in ``elm327_panic()``
-- DMA capable rx/tx buffers
+A note on CAN bus termination
+------------------------------
-- fixup constants, constant for '``>``'
+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.
-- flushing of ``tx_work`` is too late in ``ldisc_close()``
+If communications don't work with the adapter connected, check for the
+termination resistors on its PCB and try removing them.
projects / elmcan.git / blobdiff