device_drivers/can/can327.rst

1 .. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause)
7 --------
14 -----------
21 Let's use ELM327s as CAN adapters.
26 -------------
28 This driver is an effort to turn abundant ELM327 based OBD interfaces
29 into full fledged (as far as possible) CAN interfaces.
32 the driver has to switch between its modes as quickly as possible in
33 order to fake full-duplex operation.
35 As such, can327 is a best effort driver. However, this is more than
36 enough to implement simple request-response protocols (such as OBD II),
37 and to monitor broadcast messages on a bus (such as in a vehicle).
39 Most ELM327s come as nondescript serial devices, attached via USB or
40 Bluetooth. The driver cannot recognize them by itself, and as such it
41 is up to the user to attach it in form of a TTY line discipline
44 This driver is meant for ELM327 versions 1.4b and up, see below for
50 -----------
59 ----------------------------------
61 Every ELM327 chip is factory programmed to operate at a serial setting
65 be attached on a command prompt as follows::
68            --debug \
69            --speed 38400 \
70            --eightbits \
71            --noparity \
72            --onestopbit \
73            --iflag -ICRNL,INLCR,-IXOFF \
80 Once the ldisc is attached, the CAN interface starts out unconfigured.
88 500000 bit/s is a common rate for OBD-II diagnostics.
89 If you're connecting straight to a car's OBD port, this is the speed
92 After this, you can set out as usual with candump, cansniffer, etc.
97 ------------------------------------
112 It is not indicative of their actual feature set.
118 ----------------------
120 This is a short and incomplete introduction on how to talk to an ELM327.
121 It is here to guide understanding of the controller's and the driver's
122 limitation (listed below) as well as manual testing.
127 - Command mode
128 - Reception mode
131 By default, the prompt is a "``>``", after which a command can be
139 that are only meaningful in the original OBD scenario the chip is meant
143 When a command is not recognized, such as by an older version of the
144 ELM327, a question mark is printed as a response instead of OK::
154 When a CAN frame is to be sent, the target address is configured, after
155 which the frame is sent as a command that consists of the data's hex
170 Once a frame has been sent and wait-for-reply mode is on (``ATR1``,
171 configured on ``listen-only=off``), or when the reply timeout expires
178 For EFF (29 bit) CAN frames, the address format is slightly different,
183 The ELM327 will receive both SFF and EFF frames - the current CAN
190 error message may appear on the same line as the last (usually
198 ------------------------------------
200 - Clone devices ("v1.5" and others)
202   Sending RTR frames is not supported and will be dropped silently.
207   "``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is
208   not supported, and is hard coded to ON. Thus, frames are not ACKed
212   Once the bus goes silent, or an error occurs (such as BUFFER FULL),
221 - All versions
223   No full duplex operation is supported. The driver will switch
224   between input/output mode as quickly as possible.
227   clones (tested with one identifying as "``v1.5``") are unable to
230   We don't have a way to get real-time notifications on CAN errors.
231   While there is a command (``AT CS``) to retrieve some basic stats,
232   we don't poll it as it would force us to interrupt reception mode.
235 - Versions prior to 1.4b
240   controller as useful as possible.
243   command, and the "listen-only" CAN option will take effect.
246 - Versions prior to 1.4
249   change bitrate or SFF/EFF mode on-the-fly. This will have to be
254 - Versions prior to 1.3
257   the "``AT D1``" command, which is necessary to avoid parsing conflicts
258   on incoming data, as well as distinction of RTR frame lengths.
261   frames, and to check whether frames are complete. While it is possible
270 --------------------------------
272 - No 8/7 timing.
275   n is an integer divisor.
276   However there is an exception: With a separate flag, it may set the
278   This mode is not currently implemented.
280 - No evaluation of command responses.
282   The ELM327 will reply with OK when a command is understood, and with ?
283   when it is not. The driver does not currently check this, and simply
285   The driver is built such that functionality degrades gracefully
288 - No use of hardware CAN ID filtering
299 ------------------------------------------
313   but this fails if the line is not transmitted fully to
324 ------------------------------
327 the bus. This is correct when it is plugged into a OBD-II socket, but