1 .. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
3 can327: ELM327 driver for Linux SocketCAN
4 ==========================================
9 This is the non-upstreamed version of the can327 driver.
10 Please see out-of-tree.rst for compilation/usage hints.
17 Max Staudt <max-linux@enpas.org>
24 This driver aims to lower the initial cost for hackers interested in
25 working with CAN buses.
27 CAN adapters are expensive, few, and far between.
28 ELM327 interfaces are cheap and plentiful.
29 Let's use ELM327s as CAN adapters.
36 This driver is an effort to turn abundant ELM327 based OBD interfaces
37 into full fledged (as far as possible) CAN interfaces.
39 Since the ELM327 was never meant to be a stand alone CAN controller,
40 the driver has to switch between its modes as quickly as possible in
41 order to fake full-duplex operation.
43 As such, can327 is a best effort driver. However, this is more than
44 enough to implement simple request-response protocols (such as OBD II),
45 and to monitor broadcast messages on a bus (such as in a vehicle).
47 Most ELM327s come as nondescript serial devices, attached via USB or
48 Bluetooth. The driver cannot recognize them by itself, and as such it
49 is up to the user to attach it in form of a TTY line discipline
50 (similar to PPP, SLIP, slcan, ...).
52 This driver is meant for ELM327 versions 1.4b and up, see below for
53 known limitations in older controllers and clones.
60 The official data sheets can be found at ELM electronics' home page:
62 https://www.elmelectronics.com/
66 How to attach the line discipline
67 ----------------------------------
69 Every ELM327 chip is factory programmed to operate at a serial setting
70 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
72 If you have kept this default configuration, the line discipline can
73 be attached on a command prompt as follows::
81 --iflag -ICRNL,INLCR,-IXOFF \
85 To change the ELM327's serial settings, please refer to its data
86 sheet. This needs to be done before attaching the line discipline.
88 Once the ldisc is attached, the CAN interface starts out unconfigured.
89 Set the speed before starting it::
91 # The interface needs to be down to change parameters
92 sudo ip link set can0 down
93 sudo ip link set can0 type can bitrate 500000
94 sudo ip link set can0 up
96 500000 bit/s is a common rate for OBD-II diagnostics.
97 If you're connecting straight to a car's OBD port, this is the speed
98 that most cars (but not all!) expect.
100 After this, you can set out as usual with candump, cansniffer, etc.
104 How to check the controller version
105 ------------------------------------
107 Use a terminal program to attach to the controller.
109 After issuing the "``AT WS``" command, the controller will respond with
119 Note that clones may claim to be any version they like.
120 It is not indicative of their actual feature set.
125 Communication example
126 ----------------------
128 This is a short and incomplete introduction on how to talk to an ELM327.
129 It is here to guide understanding of the controller's and the driver's
130 limitation (listed below) as well as manual testing.
133 The ELM327 has two modes:
138 In command mode, it expects one command per line, terminated by CR.
139 By default, the prompt is a "``>``", after which a command can be
146 The init script in the driver switches off several configuration options
147 that are only meaningful in the original OBD scenario the chip is meant
148 for, and are actually a hindrance for can327.
151 When a command is not recognized, such as by an older version of the
152 ELM327, a question mark is printed as a response instead of OK::
158 At present, can327 does not evaluate this response and silently assumes
159 that all commands are recognized. It is structured such that it will
160 degrade gracefully when a command is unknown. See the sections above on
161 known limitations for details.
164 When a CAN frame is to be sent, the target address is configured, after
165 which the frame is sent as a command that consists of the data's hex
174 The above interaction sends the SFF frame "``DE AD BE EF 12 34 56 78``"
175 with (11 bit) CAN ID ``0x123``.
176 For this to function, the controller must be configured for SFF sending
177 mode (using "``AT PB``", see code or datasheet).
180 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
181 configured on ``listen-only=off``), or when the reply timeout expires
182 and the driver sets the controller into monitoring mode (``ATMA``),
183 the ELM327 will send one line for each received CAN frame, consisting
184 of CAN ID, DLC, and data::
186 123 8 DEADBEEF12345678
188 For EFF (29 bit) CAN frames, the address format is slightly different,
189 which can327 uses to tell the two apart::
191 12 34 56 78 8 DEADBEEF12345678
193 The ELM327 will receive both SFF and EFF frames - the current CAN
194 config (``ATPB``) does not matter.
197 If the ELM327's internal UART sending buffer runs full, it will abort
198 the monitoring mode, print "BUFFER FULL" and drop back into command
199 mode. Note that in this case, unlike with other error messages, the
200 error message may appear on the same line as the last (usually
201 incomplete) data frame::
203 12 34 56 78 8 DEADBEEF123 BUFFER FULL
207 Known limitations of the controller
208 ------------------------------------
210 - Clone devices ("v1.5" and others)
212 Sending RTR frames is not supported and will be dropped silently.
214 Receiving RTR with DLC 8 will appear to be a regular frame with
215 the last received frame's DLC and payload.
217 "``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is
218 not supported, and is hard coded to ON. Thus, frames are not ACKed
219 while listening: "``AT MA``" (Monitor All) will always be "silent".
220 However, immediately after sending a frame, the ELM327 will be in
221 "receive reply" mode, in which it *does* ACK any received frames.
222 Once the bus goes silent, or an error occurs (such as BUFFER FULL),
223 or the receive reply timeout runs out, the ELM327 will end reply
224 reception mode on its own and can327 will fall back to "``AT MA``"
225 in order to keep monitoring the bus.
227 Other limitations may apply, depending on the clone and the quality
233 No full duplex operation is supported. The driver will switch
234 between input/output mode as quickly as possible.
236 The length of outgoing RTR frames cannot be set. In fact, some
237 clones (tested with one identifying as "``v1.5``") are unable to
238 send RTR frames at all.
240 We don't have a way to get real-time notifications on CAN errors.
241 While there is a command (``AT CS``) to retrieve some basic stats,
242 we don't poll it as it would force us to interrupt reception mode.
245 - Versions prior to 1.4b
247 These versions do not send CAN ACKs when in monitoring mode (AT MA).
248 However, they do send ACKs while waiting for a reply immediately
249 after sending a frame. The driver maximizes this time to make the
250 controller as useful as possible.
252 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
253 command, and the "listen-only" CAN option will take effect.
256 - Versions prior to 1.4
258 These chips do not support the "``AT PB``" command, and thus cannot
259 change bitrate or SFF/EFF mode on-the-fly. This will have to be
260 programmed by the user before attaching the line discipline. See the
261 data sheet for details.
264 - Versions prior to 1.3
266 These chips cannot be used at all with can327. They do not support
267 the "``AT D1``" command, which is necessary to avoid parsing conflicts
268 on incoming data, as well as distinction of RTR frame lengths.
270 Specifically, this allows for easy distinction of SFF and EFF
271 frames, and to check whether frames are complete. While it is possible
272 to deduce the type and length from the length of the line the ELM327
273 sends us, this method fails when the ELM327's UART output buffer
274 overruns. It may abort sending in the middle of the line, which will
275 then be mistaken for something else.
279 Known limitations of the driver
280 --------------------------------
284 ELM327 can only set CAN bitrates that are of the form 500000/n, where
285 n is an integer divisor.
286 However there is an exception: With a separate flag, it may set the
287 speed to be 8/7 of the speed indicated by the divisor.
288 This mode is not currently implemented.
290 - No evaluation of command responses.
292 The ELM327 will reply with OK when a command is understood, and with ?
293 when it is not. The driver does not currently check this, and simply
294 assumes that the chip understands every command.
295 The driver is built such that functionality degrades gracefully
296 nevertheless. See the section on known limitations of the controller.
298 - No use of hardware CAN ID filtering
300 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
301 load, resulting in the "``BUFFER FULL``" message. Using the hardware
302 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
303 helpful here, however SocketCAN does not currently provide a facility
304 to make use of such hardware features.
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.