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 asa 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 The official data sheets can be found at ELM electronics' home page:
51 https://www.elmelectronics.com/
55 How to check the controller version
56 ------------------------------------
58 Use a terminal program to attach to the controller.
60 After issuing the "``AT WS``" command, the controller will respond with
72 How to attach the line discipline
73 ----------------------------------
75 Every ELM327 chip is factory programmed to operate at a serial setting
76 of 38400 baud/s, 8 data bits, no parity, 1 stopbit.
78 The line discipline can be attached on a command prompt as follows::
86 --iflag -ICRNL,INLCR,-IXOFF \
90 To change the ELM327's serial settings, please refer to its data
91 sheet. This needs to be done before attaching the line discipline.
98 - ``accept_flaky_uart`` - Try to live with a bad controller or UART line
100 Some adapters and/or their connection are unreliable. Enable this
101 option to try and work around the situation. This is a best-effort
102 workaround, so undefined behavior will likely occur sooner or later.
106 module/kernel parameter: accept_flaky_uart=[0|1]
110 Known limitations of the controller
111 ------------------------------------
113 - Clone "v1.5" devices
115 Sending RTR frames is not supported and will be dropped silently.
117 Receiving RTR with DLC 8 will appear to be a regular frame with
118 the last received frame's DLC and payload.
120 "``AT CSM``" not supported, thus no ACK-ing frames while listening:
121 "``AT MA``" will always be silent. However, immediately after
122 sending a frame, the ELM327 will be in "receive reply" mode, in
123 which it *does* ACK any received frames. Once the bus goes silent
124 or an error occurs (such as BUFFER FULL), the ELM327 will end reply
125 reception mode on its own and elmcan will fall back to "``AT MA``"
126 in order to keep monitoring the bus.
131 No automatic full duplex operation is supported. The driver will
132 switch between input/output mode as quickly as possible.
134 The length of outgoing RTR frames cannot be set. In fact, some
135 clones (tested with one identifying as "``v1.5``") are unable to
136 send RTR frames at all.
138 We don't have a way to get real-time notifications on CAN errors.
139 While there is a command (``AT CS``) to retrieve some basic stats,
140 we don't poll it as it would force us to interrupt reception mode.
143 - Versions prior to 1.4b
145 These versions do not send CAN ACKs when in monitoring mode (AT MA).
146 However, they do send ACKs while waiting for a reply immediately
147 after sending a frame. The driver maximizes this time to make the
148 controller as useful as possible.
150 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
151 command, and the "listen-only" CAN option will take effect.
154 - Versions prior to 1.4
156 These chips do not support the "``AT PB``" command, and thus cannot
157 change bitrate or SFF/EFF mode on-the-fly. This will have to be
158 programmed by the user before attaching the line discipline. See the
159 data sheet for details.
162 - Versions prior to 1.3
164 These chips cannot be used at all with elmcan. They do not support
165 the "``AT D1``", which is necessary to avoid parsing conflicts on
166 incoming data, as well as distinction of RTR frame lengths.
168 Specifically, this allows for easy distinction of SFF and EFF
169 frames, and to check whether frames are complete. While it is possible
170 to deduce the type and length from the length of the line the ELM327
171 sends us, this method fails when the ELM327's UART output buffer
172 overruns. It may abort sending in the middle of the line, which will
173 then be mistaken for something else.
177 Known limitations of the driver
178 --------------------------------
182 ELM327 can only set CAN bitrates that are of the form 500000/n, where
183 n is an integer divisor.
184 However there is an exception: With a separate flag, it may set the
185 speed to be 8/7 of the speed indicated by the divisor.
186 This mode is not currently implemented.
188 - No evaluation of command responses.
190 The ELM327 will reply with OK when a command is understood, and with ?
191 when it is not. The driver does not currently check this, and simply
192 assumes that the chip understands every command.
193 The driver is built such that functionality degrades gracefully
194 nevertheless. See the section on known limitations of the controller.
196 - No use of hardware CAN ID filtering
198 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
199 load, resulting in the "``BUFFER FULL``" message. Using the hardware
200 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
201 helpful here, however SocketCAN does not currently provide a facility
202 to make use of such hardware features.
206 Communication example
207 ----------------------
209 This is a short and incomplete introduction on how to talk to an ELM327.
212 The ELM327 has two modes:
217 In command mode, it expects one command per line, terminated by CR.
218 By default, the prompt is a "``>``", after which a command can be
225 The init script in the driver switches off several configuration options
226 that are only meaningful in the original OBD scenario the chip is meant
227 for, and are actually a hindrance for elmcan.
230 When a command is not recognized, such as by an older version of the
231 ELM327, a question mark is printed as a response instead of OK::
237 At present, elmcan does not evaluate this response and silently assumes
238 that all commands are recognized. It is structured such that it will
239 degrade gracefully when a command is unknown. See the sections above on
240 known limitations for details.
243 When a CAN frame is to be sent, the target address is configured, after
244 which the frame is sent as a command that consists of the data's hex
253 The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
254 the 11 bit CAN ID ``0x123``.
255 For this to function, the controller must be configured for 11 bit CAN
256 ID sending mode (using "``AT PB``", see code or datasheet).
259 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
260 configured on ``listen-only=off``), or when the reply timeout expires and
261 the driver sets the controller into monitoring mode (``ATMA``), the ELM327
262 will send one line for each received CAN frame, consisting of CAN ID,
265 123 8 DEADBEEF12345678
267 For 29 bit CAN frames, the address format is slightly different, which
268 elmcan uses to tell the two apart::
270 12 34 56 78 8 DEADBEEF12345678
272 The ELM327 will receive both 11 and 29 bit frames - the current CAN
273 config (``ATPB``) does not matter.
276 If the ELM327's internal UART sending buffer runs full, it will abort
277 the monitoring mode, print "BUFFER FULL" and drop back into command
278 mode. Note that in this case, unlike with other error messages, the
279 error message may appear on the same line as the last (usually
280 incomplete) data frame::
282 12 34 56 78 8 DEADBEEF123 BUFFER FULL
286 Rationale behind the chosen configuration
287 ------------------------------------------
292 We need this to be able to get a prompt reliably.
297 We need this to distinguish 11/29 bit CAN addresses received.
300 We can usually do this using the line length (odd/even),
301 but this fails if the line is not transmitted fully to
302 the host (BUFFER FULL).
307 We need this to tell the "length" of RTR frames.
311 To Do list for future development
312 ----------------------------------
314 - DMA capable rx/tx buffers
316 - flushing of ``tx_work`` is too late in ``ldisc_close()``