1 Linux SocketCAN driver for ELM327
2 ==================================
7 Max Staudt <max-linux@enpas.org>
14 CAN adapters are expensive, few, and far between.
15 ELM327 interfaces are cheap and plentiful.
17 This driver aims to lower the initial cost for hackers interested in
18 working with CAN buses.
25 This driver is an effort to turn abundant ELM327 based OBD interfaces
26 into full-fledged (as far as possible) CAN interfaces.
28 Since the ELM327 was never meant to be a stand-alone CAN controller,
29 the driver has to switch between its modes as quickly as possible in
30 order to approximate full-duplex operation.
32 As such, elmcan is a best-effort driver. However, this is more than
33 enough to implement simple request-response protocols (such as OBD II),
34 and to monitor broadcast messages on a bus (such as in a vehicle).
36 Most ELM327s come as nondescript serial devices, attached via USB or
37 Bluetooth. The driver cannot recognize them by itself, and as such it
38 is up to the user to attach it in form of a TTY line discipline
39 (similar to PPP, SLIP, slcan, ...).
41 This driver is meant for ELM327 versions 1.4b and up, see below for
42 known limitations in older controllers and clones.
49 This requires Linux 4.11 (for 431af779256c), and has been tested on 4.19.
51 Also, elmcan depends on ``can-dev``:
60 The official data sheets can be found at ELM electronics' home page:
62 https://www.elmelectronics.com/
66 How to check the controller version
67 ------------------------------------
69 Use a terminal program to attach to the controller.
71 After issuing the "``AT WS``" command, the controller will respond with
83 How to attach the line discipline
84 ----------------------------------
86 Every ELM327 chip is factory programmed to operate at a serial setting
87 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
89 The line discipline can be attached on a command prompt as follows::
97 --iflag -ICRNL,INLCR,-IXOFF \
101 To change the ELM327's serial settings, please refer to its data
102 sheet. This needs to be done before attaching the line discipline.
109 - ``accept_flaky_uart`` - Try to live with a bad controller or UART line
111 Some adapters and/or their connection are unreliable. Enable this
112 option to try and work around the situation. This is a best-effort
113 workaround, so undefined behavior will likely occur sooner or later.
117 module/kernel parameter: accept_flaky_uart=[0|1]
121 Known limitations of the controller
122 ------------------------------------
124 - Clone "v1.5" devices
126 Sending RTR frames is not supported and will be dropped silently.
128 Receiving RTR with DLC 8 will appear to be a regular frame with
129 the last received frame's DLC and payload.
131 "``AT CSM``" not supported, thus no ACK-ing frames while listening:
132 "``AT MA``" will always be silent. However, immediately after
133 sending a frame, the ELM327 will be in "receive reply" mode, in
134 which it *does* ACK any received frames. Once the bus goes silent
135 or an error occurs (such as BUFFER FULL), the ELM327 will end reply
136 reception mode on its own and elmcan will fall back to "``AT MA``"
137 in order to keep monitoring the bus.
142 No automatic full duplex operation is supported. The driver will
143 switch between input/output mode as quickly as possible.
145 The length of outgoing RTR frames cannot be set. In fact, some
146 clones (tested with one identifying as "``v1.5``") are unable to
147 send RTR frames at all.
149 We don't have a way to get real-time notifications on CAN errors.
150 While there is a command (``AT CS``) to retrieve some basic stats,
151 we don't poll it as it would force us to interrupt reception mode.
154 - Versions prior to 1.4b
156 These versions do not send CAN ACKs when in monitoring mode (AT MA).
157 However, they do send ACKs while waiting for a reply immediately
158 after sending a frame. The driver maximizes this time to make the
159 controller as useful as possible.
161 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
162 command, and the "listen-only" CAN option will take effect.
165 - Versions prior to 1.4
167 These chips do not support the "``AT PB``" command, and thus cannot
168 change bitrate or SFF/EFF mode on-the-fly. This will have to be
169 programmed by the user before attaching the line discipline. See the
170 data sheet for details.
173 - Versions prior to 1.3
175 These chips cannot be used at all with elmcan. They do not support
176 the "``AT D1``", which is necessary to avoid parsing conflicts on
177 incoming data, as well as distinction of RTR frame lengths.
179 Specifically, this allows for easy distinction of SFF and EFF
180 frames, and to check whether frames are complete. While it is possible
181 to deduce the type and length from the length of the line the ELM327
182 sends us, this method fails when the ELM327's UART output buffer
183 overruns. It may abort sending in the middle of the line, which will
184 then be mistaken for something else.
188 Known limitations of the driver
189 --------------------------------
193 ELM327 can only set CAN bitrates that are of the form 500000/n, where
194 n is an integer divisor.
195 However there is an exception: With a separate flag, it may set the
196 speed to be 8/7 of the speed indicated by the divisor.
197 This mode is not currently implemented.
199 - No evaluation of command responses.
201 The ELM327 will reply with OK when a command is understood, and with ?
202 when it is not. The driver does not currently check this, and simply
203 assumes that the chip understands every command.
204 The driver is built such that functionality degrades gracefully
205 nevertheless. See the section on known limitations of the controller.
207 - No use of hardware CAN ID filtering
209 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
210 load, resulting in the "``BUFFER FULL``" message. Using the hardware
211 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
212 helpful here, however SocketCAN does not currently provide a facility
213 to make use of such hardware features.
217 Communication example
218 ----------------------
220 This is a short and incomplete introduction on how to talk to an ELM327.
223 The ELM327 has two modes:
228 In command mode, it expects one command per line, terminated by CR.
229 By default, the prompt is a "``>``", after which a command can be
236 The init script in the driver switches off several configuration options
237 that are only meaningful in the original OBD scenario the chip is meant
238 for, and are actually a hindrance for elmcan.
241 When a command is not recognized, such as by an older version of the
242 ELM327, a question mark is printed as a response instead of OK::
248 At present, elmcan does not evaluate this response and silently assumes
249 that all commands are recognized. It is structured such that it will
250 degrade gracefully when a command is unknown. See the sections above on
251 known limitations for details.
254 When a CAN frame is to be sent, the target address is configured, after
255 which the frame is sent as a command that consists of the data's hex
264 The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
265 the 11 bit CAN ID ``0x123``.
266 For this to function, the controller must be configured for 11 bit CAN
267 ID sending mode (using "``AT PB``", see code or datasheet).
270 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
271 configured on ``listen-only=off``), or when the reply timeout expires and
272 the driver sets the controller into monitoring mode (``ATMA``), the ELM327
273 will send one line for each received CAN frame, consisting of CAN ID,
276 123 8 DEADBEEF12345678
278 For 29 bit CAN frames, the address format is slightly different, which
279 elmcan uses to tell the two apart::
281 12 34 56 78 8 DEADBEEF12345678
283 The ELM327 will receive both 11 and 29 bit frames - the current CAN
284 config (``ATPB``) does not matter.
287 If the ELM327's internal UART sending buffer runs full, it will abort
288 the monitoring mode, print "BUFFER FULL" and drop back into command
289 mode. Note that in this case, unlike with other error messages, the
290 error message may appear on the same line as the last (usually
291 incomplete) data frame::
293 12 34 56 78 8 DEADBEEF123 BUFFER FULL
297 Rationale behind the chosen configuration
298 ------------------------------------------
303 We need this to be able to get a prompt reliably.
308 We need this to distinguish 11/29 bit CAN addresses received.
311 We can usually do this using the line length (odd/even),
312 but this fails if the line is not transmitted fully to
313 the host (BUFFER FULL).
318 We need this to tell the "length" of RTR frames.
322 A note on CAN bus termination
323 ------------------------------
325 Your adapter may have resistors soldered in which are meant to terminate
326 the bus. This is correct when it is plugged into a OBD-II socket, but
327 not helpful when trying to tap into the middle of an existing CAN bus.
329 If communications don't work with the adapter connected, check for the
330 termination resistors on its PCB and try removing them.
334 To Do list for future development
335 ----------------------------------
337 - DMA capable rx/tx buffers
338 (is this relevant for this driver?)
340 - flushing of ``tx_work`` is too late in ``ldisc_close()``
341 (is this still the case?)