1 ELM327 driver for Linux SocketCAN
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``:
65 The official data sheets can be found at ELM electronics' home page:
67 https://www.elmelectronics.com/
71 How to check the controller version
72 ------------------------------------
74 Use a terminal program to attach to the controller.
75 The default settings are 38400 baud/s, 8 data bits, no parity, 1 stopbit.
77 After issuing the "``AT WS``" command, the controller will respond with
87 Note that clones may claim to be any version they like.
88 It is not indicative of their actual feature set.
92 How to attach the line discipline
93 ----------------------------------
95 Every ELM327 chip is factory programmed to operate at a serial setting
96 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
98 The line discipline can be attached on a command prompt as follows::
106 --iflag -ICRNL,INLCR,-IXOFF \
110 To change the ELM327's serial settings, please refer to its data
111 sheet. This needs to be done before attaching the line discipline.
118 - ``accept_flaky_uart`` - Try to live with a bad controller or UART line
120 Some adapters and/or their connection are unreliable. Enable this
121 option to try and work around the situation. This is a best effort
122 workaround, so undefined behavior will likely occur sooner or later.
123 Without this option, the driver will bail on the first unrecognized
124 character it receives from the TTY.
128 module/kernel parameter: accept_flaky_uart=[0|1]
132 Known limitations of the controller
133 ------------------------------------
135 - Clone devices ("v1.5" and others)
137 Sending RTR frames is not supported and will be dropped silently.
139 Receiving RTR with DLC 8 will appear to be a regular frame with
140 the last received frame's DLC and payload.
142 "``AT CSM``" not supported, thus no ACK-ing frames while listening:
143 "``AT MA``" will always be silent. However, immediately after
144 sending a frame, the ELM327 will be in "receive reply" mode, in
145 which it *does* ACK any received frames. Once the bus goes silent
146 or an error occurs (such as BUFFER FULL), the ELM327 will end reply
147 reception mode on its own and elmcan will fall back to "``AT MA``"
148 in order to keep monitoring the bus.
153 No full duplex operation is supported. The driver will switch
154 between input/output mode as quickly as possible.
156 The length of outgoing RTR frames cannot be set. In fact, some
157 clones (tested with one identifying as "``v1.5``") are unable to
158 send RTR frames at all.
160 We don't have a way to get real-time notifications on CAN errors.
161 While there is a command (``AT CS``) to retrieve some basic stats,
162 we don't poll it as it would force us to interrupt reception mode.
165 - Versions prior to 1.4b
167 These versions do not send CAN ACKs when in monitoring mode (AT MA).
168 However, they do send ACKs while waiting for a reply immediately
169 after sending a frame. The driver maximizes this time to make the
170 controller as useful as possible.
172 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
173 command, and the "listen-only" CAN option will take effect.
176 - Versions prior to 1.4
178 These chips do not support the "``AT PB``" command, and thus cannot
179 change bitrate or SFF/EFF mode on-the-fly. This will have to be
180 programmed by the user before attaching the line discipline. See the
181 data sheet for details.
184 - Versions prior to 1.3
186 These chips cannot be used at all with elmcan. They do not support
187 the "``AT D1``", which is necessary to avoid parsing conflicts on
188 incoming data, as well as distinction of RTR frame lengths.
190 Specifically, this allows for easy distinction of SFF and EFF
191 frames, and to check whether frames are complete. While it is possible
192 to deduce the type and length from the length of the line the ELM327
193 sends us, this method fails when the ELM327's UART output buffer
194 overruns. It may abort sending in the middle of the line, which will
195 then be mistaken for something else.
199 Known limitations of the driver
200 --------------------------------
204 ELM327 can only set CAN bitrates that are of the form 500000/n, where
205 n is an integer divisor.
206 However there is an exception: With a separate flag, it may set the
207 speed to be 8/7 of the speed indicated by the divisor.
208 This mode is not currently implemented.
210 - No evaluation of command responses.
212 The ELM327 will reply with OK when a command is understood, and with ?
213 when it is not. The driver does not currently check this, and simply
214 assumes that the chip understands every command.
215 The driver is built such that functionality degrades gracefully
216 nevertheless. See the section on known limitations of the controller.
218 - No use of hardware CAN ID filtering
220 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
221 load, resulting in the "``BUFFER FULL``" message. Using the hardware
222 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
223 helpful here, however SocketCAN does not currently provide a facility
224 to make use of such hardware features.
228 Communication example
229 ----------------------
231 This is a short and incomplete introduction on how to talk to an ELM327.
234 The ELM327 has two modes:
239 In command mode, it expects one command per line, terminated by CR.
240 By default, the prompt is a "``>``", after which a command can be
247 The init script in the driver switches off several configuration options
248 that are only meaningful in the original OBD scenario the chip is meant
249 for, and are actually a hindrance for elmcan.
252 When a command is not recognized, such as by an older version of the
253 ELM327, a question mark is printed as a response instead of OK::
259 At present, elmcan does not evaluate this response and silently assumes
260 that all commands are recognized. It is structured such that it will
261 degrade gracefully when a command is unknown. See the sections above on
262 known limitations for details.
265 When a CAN frame is to be sent, the target address is configured, after
266 which the frame is sent as a command that consists of the data's hex
275 The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
276 the 11 bit CAN ID ``0x123``.
277 For this to function, the controller must be configured for 11 bit CAN
278 ID sending mode (using "``AT PB``", see code or datasheet).
281 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
282 configured on ``listen-only=off``), or when the reply timeout expires and
283 the driver sets the controller into monitoring mode (``ATMA``), the ELM327
284 will send one line for each received CAN frame, consisting of CAN ID,
287 123 8 DEADBEEF12345678
289 For 29 bit CAN frames, the address format is slightly different, which
290 elmcan uses to tell the two apart::
292 12 34 56 78 8 DEADBEEF12345678
294 The ELM327 will receive both 11 and 29 bit frames - the current CAN
295 config (``ATPB``) does not matter.
298 If the ELM327's internal UART sending buffer runs full, it will abort
299 the monitoring mode, print "BUFFER FULL" and drop back into command
300 mode. Note that in this case, unlike with other error messages, the
301 error message may appear on the same line as the last (usually
302 incomplete) data frame::
304 12 34 56 78 8 DEADBEEF123 BUFFER FULL
308 Rationale behind the chosen configuration
309 ------------------------------------------
314 We need this to be able to get a prompt reliably.
319 We need this to distinguish 11/29 bit CAN addresses received.
322 We can usually do this using the line length (odd/even),
323 but this fails if the line is not transmitted fully to
324 the host (BUFFER FULL).
329 We need this to tell the "length" of RTR frames.
333 A note on CAN bus termination
334 ------------------------------
336 Your adapter may have resistors soldered in which are meant to terminate
337 the bus. This is correct when it is plugged into a OBD-II socket, but
338 not helpful when trying to tap into the middle of an existing CAN bus.
340 If communications don't work with the adapter connected, check for the
341 termination resistors on its PCB and try removing them.
348 Thanks go out to Oliver Neukum for his early reviews and suggestions.
350 Several more people have encouraged me to finish this - thank you all.