1 Linux SocketCAN driver for ELM327
2 ==================================
7 Max Staudt <max-linux@enpas.org>
14 This driver aims to lower the initial cost for hackers interested in
15 working with CAN buses.
17 CAN adapters are expensive, few, and far between.
18 ELM327 interfaces are cheap and plentiful.
19 Let's use ELM327s as CAN adapters.
26 This driver is an effort to turn abundant ELM327 based OBD interfaces
27 into full-fledged (as far as possible) CAN interfaces.
29 Since the ELM327 was never meant to be a stand-alone CAN controller,
30 the driver has to switch between its modes as quickly as possible in
31 order to fake full-duplex operation.
33 As such, elmcan is a best-effort driver. However, this is more than
34 enough to implement simple request-response protocols (such as OBD II),
35 and to monitor broadcast messages on a bus (such as in a vehicle).
37 Most ELM327s come as nondescript serial devices, attached via USB or
38 Bluetooth. The driver cannot recognize them by itself, and as such it
39 is up to the user to attach it in form of a TTY line discipline
40 (similar to PPP, SLIP, slcan, ...).
42 This driver is meant for ELM327 versions 1.4b and up, see below for
43 known limitations in older controllers and clones.
50 This requires Linux 4.11 (for 431af779256c), and has been tested on 4.19.
52 Also, elmcan depends on ``can-dev``:
61 The official data sheets can be found at ELM electronics' home page:
63 https://www.elmelectronics.com/
67 How to check the controller version
68 ------------------------------------
70 Use a terminal program to attach to the controller.
71 The default settings are 38400 baud/s, 8 data bits, no parity, 1 stopbit.
73 After issuing the "``AT WS``" command, the controller will respond with
83 Note that clones may claim to be any version they like.
84 It is not indicative of their actual feature set.
88 How to attach the line discipline
89 ----------------------------------
91 Every ELM327 chip is factory programmed to operate at a serial setting
92 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
94 The line discipline can be attached on a command prompt as follows::
102 --iflag -ICRNL,INLCR,-IXOFF \
106 To change the ELM327's serial settings, please refer to its data
107 sheet. This needs to be done before attaching the line discipline.
114 - ``accept_flaky_uart`` - Try to live with a bad controller or UART line
116 Some adapters and/or their connection are unreliable. Enable this
117 option to try and work around the situation. This is a best-effort
118 workaround, so undefined behavior will likely occur sooner or later.
119 Without this option, the driver will bail on the first unrecognized
120 character it receives from the TTY.
124 module/kernel parameter: accept_flaky_uart=[0|1]
128 Known limitations of the controller
129 ------------------------------------
131 - Clone devices ("v1.5" and others)
133 Sending RTR frames is not supported and will be dropped silently.
135 Receiving RTR with DLC 8 will appear to be a regular frame with
136 the last received frame's DLC and payload.
138 "``AT CSM``" not supported, thus no ACK-ing frames while listening:
139 "``AT MA``" will always be silent. However, immediately after
140 sending a frame, the ELM327 will be in "receive reply" mode, in
141 which it *does* ACK any received frames. Once the bus goes silent
142 or an error occurs (such as BUFFER FULL), the ELM327 will end reply
143 reception mode on its own and elmcan will fall back to "``AT MA``"
144 in order to keep monitoring the bus.
149 No full duplex operation is supported. The driver will switch
150 between input/output mode as quickly as possible.
152 The length of outgoing RTR frames cannot be set. In fact, some
153 clones (tested with one identifying as "``v1.5``") are unable to
154 send RTR frames at all.
156 We don't have a way to get real-time notifications on CAN errors.
157 While there is a command (``AT CS``) to retrieve some basic stats,
158 we don't poll it as it would force us to interrupt reception mode.
161 - Versions prior to 1.4b
163 These versions do not send CAN ACKs when in monitoring mode (AT MA).
164 However, they do send ACKs while waiting for a reply immediately
165 after sending a frame. The driver maximizes this time to make the
166 controller as useful as possible.
168 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
169 command, and the "listen-only" CAN option will take effect.
172 - Versions prior to 1.4
174 These chips do not support the "``AT PB``" command, and thus cannot
175 change bitrate or SFF/EFF mode on-the-fly. This will have to be
176 programmed by the user before attaching the line discipline. See the
177 data sheet for details.
180 - Versions prior to 1.3
182 These chips cannot be used at all with elmcan. They do not support
183 the "``AT D1``", which is necessary to avoid parsing conflicts on
184 incoming data, as well as distinction of RTR frame lengths.
186 Specifically, this allows for easy distinction of SFF and EFF
187 frames, and to check whether frames are complete. While it is possible
188 to deduce the type and length from the length of the line the ELM327
189 sends us, this method fails when the ELM327's UART output buffer
190 overruns. It may abort sending in the middle of the line, which will
191 then be mistaken for something else.
195 Known limitations of the driver
196 --------------------------------
200 ELM327 can only set CAN bitrates that are of the form 500000/n, where
201 n is an integer divisor.
202 However there is an exception: With a separate flag, it may set the
203 speed to be 8/7 of the speed indicated by the divisor.
204 This mode is not currently implemented.
206 - No evaluation of command responses.
208 The ELM327 will reply with OK when a command is understood, and with ?
209 when it is not. The driver does not currently check this, and simply
210 assumes that the chip understands every command.
211 The driver is built such that functionality degrades gracefully
212 nevertheless. See the section on known limitations of the controller.
214 - No use of hardware CAN ID filtering
216 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
217 load, resulting in the "``BUFFER FULL``" message. Using the hardware
218 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
219 helpful here, however SocketCAN does not currently provide a facility
220 to make use of such hardware features.
224 Communication example
225 ----------------------
227 This is a short and incomplete introduction on how to talk to an ELM327.
230 The ELM327 has two modes:
235 In command mode, it expects one command per line, terminated by CR.
236 By default, the prompt is a "``>``", after which a command can be
243 The init script in the driver switches off several configuration options
244 that are only meaningful in the original OBD scenario the chip is meant
245 for, and are actually a hindrance for elmcan.
248 When a command is not recognized, such as by an older version of the
249 ELM327, a question mark is printed as a response instead of OK::
255 At present, elmcan does not evaluate this response and silently assumes
256 that all commands are recognized. It is structured such that it will
257 degrade gracefully when a command is unknown. See the sections above on
258 known limitations for details.
261 When a CAN frame is to be sent, the target address is configured, after
262 which the frame is sent as a command that consists of the data's hex
271 The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
272 the 11 bit CAN ID ``0x123``.
273 For this to function, the controller must be configured for 11 bit CAN
274 ID sending mode (using "``AT PB``", see code or datasheet).
277 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
278 configured on ``listen-only=off``), or when the reply timeout expires and
279 the driver sets the controller into monitoring mode (``ATMA``), the ELM327
280 will send one line for each received CAN frame, consisting of CAN ID,
283 123 8 DEADBEEF12345678
285 For 29 bit CAN frames, the address format is slightly different, which
286 elmcan uses to tell the two apart::
288 12 34 56 78 8 DEADBEEF12345678
290 The ELM327 will receive both 11 and 29 bit frames - the current CAN
291 config (``ATPB``) does not matter.
294 If the ELM327's internal UART sending buffer runs full, it will abort
295 the monitoring mode, print "BUFFER FULL" and drop back into command
296 mode. Note that in this case, unlike with other error messages, the
297 error message may appear on the same line as the last (usually
298 incomplete) data frame::
300 12 34 56 78 8 DEADBEEF123 BUFFER FULL
304 Rationale behind the chosen configuration
305 ------------------------------------------
310 We need this to be able to get a prompt reliably.
315 We need this to distinguish 11/29 bit CAN addresses received.
318 We can usually do this using the line length (odd/even),
319 but this fails if the line is not transmitted fully to
320 the host (BUFFER FULL).
325 We need this to tell the "length" of RTR frames.
329 A note on CAN bus termination
330 ------------------------------
332 Your adapter may have resistors soldered in which are meant to terminate
333 the bus. This is correct when it is plugged into a OBD-II socket, but
334 not helpful when trying to tap into the middle of an existing CAN bus.
336 If communications don't work with the adapter connected, check for the
337 termination resistors on its PCB and try removing them.
341 To Do list for future development
342 ----------------------------------
344 - flushing of ``tx_work`` is too late in ``ldisc_close()``
345 (is this still the case?)