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diff --git a/Documentation/networking/device_drivers/can/can327.rst b/Documentation/networking/device_drivers/can/can327.rst new file mode 100644 index 000000000..b87bfbe5d --- /dev/null +++ b/Documentation/networking/device_drivers/can/can327.rst @@ -0,0 +1,331 @@ +.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause) + +can327: ELM327 driver for Linux SocketCAN +========================================== + +Authors +-------- + +Max Staudt <max@enpas.org> + + + +Motivation +----------- + +This driver aims to lower the initial cost for hackers interested in +working with CAN buses. + +CAN adapters are expensive, few, and far between. +ELM327 interfaces are cheap and plentiful. +Let's use ELM327s as CAN adapters. + + + +Introduction +------------- + +This driver is an effort to turn abundant ELM327 based OBD interfaces +into full fledged (as far as possible) CAN interfaces. + +Since the ELM327 was never meant to be a stand alone CAN controller, +the driver has to switch between its modes as quickly as possible in +order to fake full-duplex operation. + +As such, can327 is a best effort driver. However, this is more than +enough to implement simple request-response protocols (such as OBD II), +and to monitor broadcast messages on a bus (such as in a vehicle). + +Most ELM327s come as nondescript serial devices, attached via USB or +Bluetooth. The driver cannot recognize them by itself, and as such it +is up to the user to attach it in form of a TTY line discipline +(similar to PPP, SLIP, slcan, ...). + +This driver is meant for ELM327 versions 1.4b and up, see below for +known limitations in older controllers and clones. + + + +Data sheet +----------- + +The official data sheets can be found at ELM electronics' home page: + + https://www.elmelectronics.com/ + + + +How to attach the line discipline +---------------------------------- + +Every ELM327 chip is factory programmed to operate at a serial setting +of 38400 baud/s, 8 data bits, no parity, 1 stopbit. + +If you have kept this default configuration, the line discipline can +be attached on a command prompt as follows:: + + sudo ldattach \ + --debug \ + --speed 38400 \ + --eightbits \ + --noparity \ + --onestopbit \ + --iflag -ICRNL,INLCR,-IXOFF \ + 30 \ + /dev/ttyUSB0 + +To change the ELM327's serial settings, please refer to its data +sheet. This needs to be done before attaching the line discipline. + +Once the ldisc is attached, the CAN interface starts out unconfigured. +Set the speed before starting it:: + + # The interface needs to be down to change parameters + sudo ip link set can0 down + sudo ip link set can0 type can bitrate 500000 + sudo ip link set can0 up + +500000 bit/s is a common rate for OBD-II diagnostics. +If you're connecting straight to a car's OBD port, this is the speed +that most cars (but not all!) expect. + +After this, you can set out as usual with candump, cansniffer, etc. + + + +How to check the controller version +------------------------------------ + +Use a terminal program to attach to the controller. + +After issuing the "``AT WS``" command, the controller will respond with +its version:: + + >AT WS + + + ELM327 v1.4b + + > + +Note that clones may claim to be any version they like. +It is not indicative of their actual feature set. + + + + +Communication example +---------------------- + +This is a short and incomplete introduction on how to talk to an ELM327. +It is here to guide understanding of the controller's and the driver's +limitation (listed below) as well as manual testing. + + +The ELM327 has two modes: + +- Command mode +- Reception mode + +In command mode, it expects one command per line, terminated by CR. +By default, the prompt is a "``>``", after which a command can be +entered:: + + >ATE1 + OK + > + +The init script in the driver switches off several configuration options +that are only meaningful in the original OBD scenario the chip is meant +for, and are actually a hindrance for can327. + + +When a command is not recognized, such as by an older version of the +ELM327, a question mark is printed as a response instead of OK:: + + >ATUNKNOWN + ? + > + +At present, can327 does not evaluate this response. See the section +below on known limitations for details. + + +When a CAN frame is to be sent, the target address is configured, after +which the frame is sent as a command that consists of the data's hex +dump:: + + >ATSH123 + OK + >DEADBEEF12345678 + OK + > + +The above interaction sends the SFF frame "``DE AD BE EF 12 34 56 78``" +with (11 bit) CAN ID ``0x123``. +For this to function, the controller must be configured for SFF sending +mode (using "``AT PB``", see code or datasheet). + + +Once a frame has been sent and wait-for-reply mode is on (``ATR1``, +configured on ``listen-only=off``), or when the reply timeout expires +and the driver sets the controller into monitoring mode (``ATMA``), +the ELM327 will send one line for each received CAN frame, consisting +of CAN ID, DLC, and data:: + + 123 8 DEADBEEF12345678 + +For EFF (29 bit) CAN frames, the address format is slightly different, +which can327 uses to tell the two apart:: + + 12 34 56 78 8 DEADBEEF12345678 + +The ELM327 will receive both SFF and EFF frames - the current CAN +config (``ATPB``) does not matter. + + +If the ELM327's internal UART sending buffer runs full, it will abort +the monitoring mode, print "BUFFER FULL" and drop back into command +mode. Note that in this case, unlike with other error messages, the +error message may appear on the same line as the last (usually +incomplete) data frame:: + + 12 34 56 78 8 DEADBEEF123 BUFFER FULL + + + +Known limitations of the controller +------------------------------------ + +- Clone devices ("v1.5" and others) + + Sending RTR frames is not supported and will be dropped silently. + + Receiving RTR with DLC 8 will appear to be a regular frame with + the last received frame's DLC and payload. + + "``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is + not supported, and is hard coded to ON. Thus, frames are not ACKed + while listening: "``AT MA``" (Monitor All) will always be "silent". + However, immediately after sending a frame, the ELM327 will be in + "receive reply" mode, in which it *does* ACK any received frames. + Once the bus goes silent, or an error occurs (such as BUFFER FULL), + or the receive reply timeout runs out, the ELM327 will end reply + reception mode on its own and can327 will fall back to "``AT MA``" + in order to keep monitoring the bus. + + Other limitations may apply, depending on the clone and the quality + of its firmware. + + +- All versions + + No full duplex operation is supported. The driver will switch + between input/output mode as quickly as possible. + + The length of outgoing RTR frames cannot be set. In fact, some + clones (tested with one identifying as "``v1.5``") are unable to + send RTR frames at all. + + We don't have a way to get real-time notifications on CAN errors. + While there is a command (``AT CS``) to retrieve some basic stats, + we don't poll it as it would force us to interrupt reception mode. + + +- Versions prior to 1.4b + + These versions do not send CAN ACKs when in monitoring mode (AT MA). + However, they do send ACKs while waiting for a reply immediately + after sending a frame. The driver maximizes this time to make the + controller as useful as possible. + + Starting with version 1.4b, the ELM327 supports the "``AT CSM``" + command, and the "listen-only" CAN option will take effect. + + +- Versions prior to 1.4 + + These chips do not support the "``AT PB``" command, and thus cannot + change bitrate or SFF/EFF mode on-the-fly. This will have to be + programmed by the user before attaching the line discipline. See the + data sheet for details. + + +- Versions prior to 1.3 + + These chips cannot be used at all with can327. They do not support + the "``AT D1``" command, which is necessary to avoid parsing conflicts + on incoming data, as well as distinction of RTR frame lengths. + + Specifically, this allows for easy distinction of SFF and EFF + frames, and to check whether frames are complete. While it is possible + to deduce the type and length from the length of the line the ELM327 + sends us, this method fails when the ELM327's UART output buffer + overruns. It may abort sending in the middle of the line, which will + then be mistaken for something else. + + + +Known limitations of the driver +-------------------------------- + +- No 8/7 timing. + + ELM327 can only set CAN bitrates that are of the form 500000/n, where + n is an integer divisor. + However there is an exception: With a separate flag, it may set the + speed to be 8/7 of the speed indicated by the divisor. + This mode is not currently implemented. + +- No evaluation of command responses. + + The ELM327 will reply with OK when a command is understood, and with ? + when it is not. The driver does not currently check this, and simply + assumes that the chip understands every command. + The driver is built such that functionality degrades gracefully + nevertheless. See the section on known limitations of the controller. + +- No use of hardware CAN ID filtering + + An ELM327's UART sending buffer will easily overflow on heavy CAN bus + load, resulting in the "``BUFFER FULL``" message. Using the hardware + filters available through "``AT CF xxx``" and "``AT CM xxx``" would be + helpful here, however SocketCAN does not currently provide a facility + to make use of such hardware features. + + + +Rationale behind the chosen configuration +------------------------------------------ + +``AT E1`` + Echo on + + We need this to be able to get a prompt reliably. + +``AT S1`` + Spaces on + + We need this to distinguish 11/29 bit CAN addresses received. + + Note: + We can usually do this using the line length (odd/even), + but this fails if the line is not transmitted fully to + the host (BUFFER FULL). + +``AT D1`` + DLC on + + We need this to tell the "length" of RTR frames. + + + +A note on CAN bus termination +------------------------------ + +Your adapter may have resistors soldered in which are meant to terminate +the bus. This is correct when it is plugged into a OBD-II socket, but +not helpful when trying to tap into the middle of an existing CAN bus. + +If communications don't work with the adapter connected, check for the +termination resistors on its PCB and try removing them. |