1 .. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
3 can327: ELM327 driver for Linux SocketCAN
4 ==========================================
9 This driver has become an official part of Linux since v6.0:
11 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/can/can327.c?h=v6.0
13 Please send patches upstream to [linux-can] and the maintainers for
16 I may or may not backport upstream patches to this downstream
17 repository, and/or use it for further pre-upstream work.
24 This is the non-upstreamed version of the can327 driver.
25 Please see out-of-tree.rst for compilation/usage hints.
32 Max Staudt <max-linux@enpas.org>
39 This driver aims to lower the initial cost for hackers interested in
40 working with CAN buses.
42 CAN adapters are expensive, few, and far between.
43 ELM327 interfaces are cheap and plentiful.
44 Let's use ELM327s as CAN adapters.
51 This driver is an effort to turn abundant ELM327 based OBD interfaces
52 into full fledged (as far as possible) CAN interfaces.
54 Since the ELM327 was never meant to be a stand alone CAN controller,
55 the driver has to switch between its modes as quickly as possible in
56 order to fake full-duplex operation.
58 As such, can327 is a best effort driver. However, this is more than
59 enough to implement simple request-response protocols (such as OBD II),
60 and to monitor broadcast messages on a bus (such as in a vehicle).
62 Most ELM327s come as nondescript serial devices, attached via USB or
63 Bluetooth. The driver cannot recognize them by itself, and as such it
64 is up to the user to attach it in form of a TTY line discipline
65 (similar to PPP, SLIP, slcan, ...).
67 This driver is meant for ELM327 versions 1.4b and up, see below for
68 known limitations in older controllers and clones.
75 The official data sheets can be found at ELM electronics' home page:
77 https://www.elmelectronics.com/
81 How to attach the line discipline
82 ----------------------------------
84 Every ELM327 chip is factory programmed to operate at a serial setting
85 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
87 If you have kept this default configuration, the line discipline can
88 be attached on a command prompt as follows::
96 --iflag -ICRNL,INLCR,-IXOFF \
100 To change the ELM327's serial settings, please refer to its data
101 sheet. This needs to be done before attaching the line discipline.
103 Once the ldisc is attached, the CAN interface starts out unconfigured.
104 Set the speed before starting it::
106 # The interface needs to be down to change parameters
107 sudo ip link set can0 down
108 sudo ip link set can0 type can bitrate 500000
109 sudo ip link set can0 up
111 500000 bit/s is a common rate for OBD-II diagnostics.
112 If you're connecting straight to a car's OBD port, this is the speed
113 that most cars (but not all!) expect.
115 After this, you can set out as usual with candump, cansniffer, etc.
119 How to check the controller version
120 ------------------------------------
122 Use a terminal program to attach to the controller.
124 After issuing the "``AT WS``" command, the controller will respond with
134 Note that clones may claim to be any version they like.
135 It is not indicative of their actual feature set.
140 Communication example
141 ----------------------
143 This is a short and incomplete introduction on how to talk to an ELM327.
144 It is here to guide understanding of the controller's and the driver's
145 limitation (listed below) as well as manual testing.
148 The ELM327 has two modes:
153 In command mode, it expects one command per line, terminated by CR.
154 By default, the prompt is a "``>``", after which a command can be
161 The init script in the driver switches off several configuration options
162 that are only meaningful in the original OBD scenario the chip is meant
163 for, and are actually a hindrance for can327.
166 When a command is not recognized, such as by an older version of the
167 ELM327, a question mark is printed as a response instead of OK::
173 At present, can327 does not evaluate this response. See the section
174 below on known limitations for details.
177 When a CAN frame is to be sent, the target address is configured, after
178 which the frame is sent as a command that consists of the data's hex
187 The above interaction sends the SFF frame "``DE AD BE EF 12 34 56 78``"
188 with (11 bit) CAN ID ``0x123``.
189 For this to function, the controller must be configured for SFF sending
190 mode (using "``AT PB``", see code or datasheet).
193 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
194 configured on ``listen-only=off``), or when the reply timeout expires
195 and the driver sets the controller into monitoring mode (``ATMA``),
196 the ELM327 will send one line for each received CAN frame, consisting
197 of CAN ID, DLC, and data::
199 123 8 DEADBEEF12345678
201 For EFF (29 bit) CAN frames, the address format is slightly different,
202 which can327 uses to tell the two apart::
204 12 34 56 78 8 DEADBEEF12345678
206 The ELM327 will receive both SFF and EFF frames - the current CAN
207 config (``ATPB``) does not matter.
210 If the ELM327's internal UART sending buffer runs full, it will abort
211 the monitoring mode, print "BUFFER FULL" and drop back into command
212 mode. Note that in this case, unlike with other error messages, the
213 error message may appear on the same line as the last (usually
214 incomplete) data frame::
216 12 34 56 78 8 DEADBEEF123 BUFFER FULL
220 Known limitations of the controller
221 ------------------------------------
223 - Clone devices ("v1.5" and others)
225 Sending RTR frames is not supported and will be dropped silently.
227 Receiving RTR with DLC 8 will appear to be a regular frame with
228 the last received frame's DLC and payload.
230 "``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is
231 not supported, and is hard coded to ON. Thus, frames are not ACKed
232 while listening: "``AT MA``" (Monitor All) will always be "silent".
233 However, immediately after sending a frame, the ELM327 will be in
234 "receive reply" mode, in which it *does* ACK any received frames.
235 Once the bus goes silent, or an error occurs (such as BUFFER FULL),
236 or the receive reply timeout runs out, the ELM327 will end reply
237 reception mode on its own and can327 will fall back to "``AT MA``"
238 in order to keep monitoring the bus.
240 Other limitations may apply, depending on the clone and the quality
246 No full duplex operation is supported. The driver will switch
247 between input/output mode as quickly as possible.
249 The length of outgoing RTR frames cannot be set. In fact, some
250 clones (tested with one identifying as "``v1.5``") are unable to
251 send RTR frames at all.
253 We don't have a way to get real-time notifications on CAN errors.
254 While there is a command (``AT CS``) to retrieve some basic stats,
255 we don't poll it as it would force us to interrupt reception mode.
258 - Versions prior to 1.4b
260 These versions do not send CAN ACKs when in monitoring mode (AT MA).
261 However, they do send ACKs while waiting for a reply immediately
262 after sending a frame. The driver maximizes this time to make the
263 controller as useful as possible.
265 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
266 command, and the "listen-only" CAN option will take effect.
269 - Versions prior to 1.4
271 These chips do not support the "``AT PB``" command, and thus cannot
272 change bitrate or SFF/EFF mode on-the-fly. This will have to be
273 programmed by the user before attaching the line discipline. See the
274 data sheet for details.
277 - Versions prior to 1.3
279 These chips cannot be used at all with can327. They do not support
280 the "``AT D1``" command, which is necessary to avoid parsing conflicts
281 on incoming data, as well as distinction of RTR frame lengths.
283 Specifically, this allows for easy distinction of SFF and EFF
284 frames, and to check whether frames are complete. While it is possible
285 to deduce the type and length from the length of the line the ELM327
286 sends us, this method fails when the ELM327's UART output buffer
287 overruns. It may abort sending in the middle of the line, which will
288 then be mistaken for something else.
292 Known limitations of the driver
293 --------------------------------
297 ELM327 can only set CAN bitrates that are of the form 500000/n, where
298 n is an integer divisor.
299 However there is an exception: With a separate flag, it may set the
300 speed to be 8/7 of the speed indicated by the divisor.
301 This mode is not currently implemented.
303 - No evaluation of command responses.
305 The ELM327 will reply with OK when a command is understood, and with ?
306 when it is not. The driver does not currently check this, and simply
307 assumes that the chip understands every command.
308 The driver is built such that functionality degrades gracefully
309 nevertheless. See the section on known limitations of the controller.
311 - No use of hardware CAN ID filtering
313 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
314 load, resulting in the "``BUFFER FULL``" message. Using the hardware
315 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
316 helpful here, however SocketCAN does not currently provide a facility
317 to make use of such hardware features.
321 Rationale behind the chosen configuration
322 ------------------------------------------
327 We need this to be able to get a prompt reliably.
332 We need this to distinguish 11/29 bit CAN addresses received.
335 We can usually do this using the line length (odd/even),
336 but this fails if the line is not transmitted fully to
337 the host (BUFFER FULL).
342 We need this to tell the "length" of RTR frames.
346 A note on CAN bus termination
347 ------------------------------
349 Your adapter may have resistors soldered in which are meant to terminate
350 the bus. This is correct when it is plugged into a OBD-II socket, but
351 not helpful when trying to tap into the middle of an existing CAN bus.
353 If communications don't work with the adapter connected, check for the
354 termination resistors on its PCB and try removing them.