1 Linux SocketCAN driver for ELM327
2 ==================================
7 Max Staudt <elmcan@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.
95 Known limitations of the controller
96 ------------------------------------
98 - Clone "v1.5" devices
100 Sending RTR frames is not supported and will be dropped silently.
102 Receiving RTR with DLC 8 will appear to be a regular frame with
103 the last received frame's DLC and payload.
105 "``AT CSM``" not supported, thus no ACK-ing frames while listening:
106 "``AT MA``" will always be silent. However, immediately after
107 sending a frame, the ELM327 will be in "receive reply" mode, in
108 which it *does* ACK any received frames. Once the bus goes silent
109 or an error occurs (such as BUFFER FULL), the ELM327 will end reply
110 reception mode on its own and elmcan will fall back to "``AT MA``"
111 in order to keep monitoring the bus.
116 No automatic full duplex operation is supported. The driver will
117 switch between input/output mode as quickly as possible.
119 The length of outgoing RTR frames cannot be set. In fact, some
120 clones (tested with one identifying as "``v1.5``") are unable to
121 send RTR frames at all.
123 We don't have a way to get real-time notifications on CAN errors.
124 While there is a command (``AT CS``) to retrieve some basic stats,
125 we don't poll it as it would force us to interrupt reception mode.
128 - Versions prior to 1.4b
130 These versions do not send CAN ACKs when in monitoring mode (AT MA).
131 However, they do send ACKs while waiting for a reply immediately
132 after sending a frame. The driver maximizes this time to make the
133 controller as useful as possible.
135 Starting with version 1.4b, the ELM327 supports the "``AT CSM``"
136 command, and the "listen-only" CAN option will take effect.
139 - Versions prior to 1.4
141 These chips do not support the "``AT PB``" command, and thus cannot
142 change bitrate or SFF/EFF mode on-the-fly. This will have to be
143 programmed by the user before attaching the line discipline. See the
144 data sheet for details.
147 - Versions prior to 1.3
149 These chips cannot be used at all with elmcan. They do not support
150 the "``AT D1``", which is necessary to avoid parsing conflicts on
151 incoming data, as well as distinction of RTR frame lengths.
153 Specifically, this allows for easy distinction of SFF and EFF
154 frames, and to check whether frames are complete. While it is possible
155 to deduce the type and length from the length of the line the ELM327
156 sends us, this method fails when the ELM327's UART output buffer
157 overruns. It may abort sending in the middle of the line, which will
158 then be mistaken for something else.
162 Known limitations of the driver
163 --------------------------------
167 ELM327 can only set CAN bitrates that are of the form 500000/n, where
168 n is an integer divisor.
169 However there is an exception: With a separate flag, it may set the
170 speed to be 8/7 of the speed indicated by the divisor.
171 This mode is not currently implemented.
173 - No evaluation of command responses.
175 The ELM327 will reply with OK when a command is understood, and with ?
176 when it is not. The driver does not currently check this, and simply
177 assumes that the chip understands every command.
178 The driver is built such that functionality degrades gracefully
179 nevertheless. See the section on known limitations of the controller.
181 - No use of hardware CAN ID filtering
183 An ELM327's UART sending buffer will easily overflow on heavy CAN bus
184 load, resulting in the "``BUFFER FULL``" message. Using the hardware
185 filters available through "``AT CF xxx``" and "``AT CM xxx``" would be
186 helpful here, however SocketCAN does not currently provide a facility
187 to make use of such hardware features.
191 Communication example
192 ----------------------
194 This is a short and incomplete introduction on how to talk to an ELM327.
197 The ELM327 has two modes:
202 In command mode, it expects one command per line, terminated by CR.
203 By default, the prompt is a "``>``", after which a command can be
210 The init script in the driver switches off several configuration options
211 that are only meaningful in the original OBD scenario the chip is meant
212 for, and are actually a hindrance for elmcan.
215 When a command is not recognized, such as by an older version of the
216 ELM327, a question mark is printed as a response instead of OK::
222 At present, elmcan does not evaluate this response and silently assumes
223 that all commands are recognized. It is structured such that it will
224 degrade gracefully when a command is unknown. See the sections above on
225 known limitations for details.
228 When a CAN frame is to be sent, the target address is configured, after
229 which the frame is sent as a command that consists of the data's hex
238 The above interaction sends the frame "``DE AD BE EF 12 34 56 78``" with
239 the 11 bit CAN ID ``0x123``.
240 For this to function, the controller must be configured for 11 bit CAN
241 ID sending mode (using "``AT PB``", see code or datasheet).
244 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
245 configured on ``listen-only=off``), or when the reply timeout expires and
246 the driver sets the controller into monitoring mode (``ATMA``), the ELM327
247 will send one line for each received CAN frame, consisting of CAN ID,
250 123 8 DEADBEEF12345678
252 For 29 bit CAN frames, the address format is slightly different, which
253 elmcan uses to tell the two apart::
255 12 34 56 78 8 DEADBEEF12345678
257 The ELM327 will receive both 11 and 29 bit frames - the current CAN
258 config (``ATPB``) does not matter.
261 If the ELM327's internal UART sending buffer runs full, it will abort
262 the monitoring mode, print "BUFFER FULL" and drop back into command
263 mode. Note that in this case, unlike with other error messages, the
264 error message may appear on the same line as the last (usually
265 incomplete) data frame::
267 12 34 56 78 8 DEADBEEF123 BUFFER FULL
271 Rationale behind the chosen configuration
272 ------------------------------------------
277 We need this to be able to get a prompt reliably.
282 We need this to distinguish 11/29 bit CAN addresses received.
285 We can usually do this using the line length (odd/even),
286 but this fails if the line is not transmitted fully to
287 the host (BUFFER FULL).
292 We need this to tell the "length" of RTR frames.
296 To Do list for future development
297 ----------------------------------
299 - DMA capable rx/tx buffers
301 - flushing of ``tx_work`` is too late in ``ldisc_close()``