ELM327芯片桥梁：OBD到RS232解释器

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"ELM327手册是一份详细介绍ELM327芯片的英文参考资料，主要应用于汽车诊断接口与标准RS232端口之间的转换，支持多种通信协议，适用于汽车爱好者和相关项目设计。" ELM327是一种集成电路，由Elm Electronics公司开发，专门设计用于解决现代汽车上的On-Board Diagnostics (OBD)接口与个人电脑或掌上设备之间不兼容的问题。根据法律要求，几乎所有的新型汽车都必须提供一个接口，以便测试设备可以获取诊断信息。然而，这些接口的数据传输遵循的标准并不直接与PC或PDA相兼容。ELM327芯片就是为了解决这个问题，作为OBD端口和标准RS232串行端口之间的桥梁。 ELM327在ELM320、ELM322和ELM323接口的基础上进行了改进，增加了对七种CAN（Controller Area Network）协议的支持。这使得该芯片能够自动检测并转换当前最常用的通信协议。除此之外，ELM327还具备高速RS232选项、电池电压监控以及通过可编程参数实现的定制功能等增强特性。使用ELM327时，需要很少的外部组件就能构建一个完整的电路。手册中详细讨论了接口细节，并指导用户如何使用该芯片与车辆进行通信。手册最后提供了两个电路图，帮助用户快速入门。 ELM327支持12伏电源，能够处理各种OBD标准，如OBD-I、OBD-II和部分制造商的专用诊断系统。通过RS232接口，用户可以使用计算机程序读取和解析汽车的诊断信息，例如故障代码、发动机性能数据等。这使得汽车维修、故障排查和性能调校变得更加便捷。 ELM327手册是理解和应用汽车诊断技术的重要资源，无论是对于汽车爱好者进行自我诊断，还是专业技师进行复杂故障分析，都是不可或缺的工具。通过深入学习和实践，用户可以更好地理解和利用ELM327的功能，提高汽车维护和修复的效率。

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ELM327

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CFC0 and CFC1 [ CAN Flow Control off or on ]

The ISO 15765-4 protocol expects a “Flow

Control” message to always be sent in response to a

“First Frame” message. The ELM327 automatically

sends these, without any intervention by the user. If

experimenting with a non-OBD system, it may be

desirable to turn this automatic response off, and the

AT CFC0 command has been provided for that

purpose. The default setting is CFC1 - Flow Controls

on.

Note that during monitoring (AT MA, MR, or MT),

there are never any Flow Controls sent no matter what

the CFC option is set to.

CM hhh [ set the CAN ID Mask to hhh ]

There can be a great many messages being

transmitted in a CAN system at any one time. In order

to limit what the ELM327 views, there needs to be a

system of filtering out the relevant ones from all the

others. This is accomplished by the filter, which works

in conjunction with the mask. A mask is a group of bits

that show the ELM327 which bits in the filter are

relevant, and which ones can be ignored. A ‘must

match’ condition is signaled by setting a mask bit to '1',

while a 'don't care' is signaled by setting a bit to '0'.

This three digit variation of the CM command is used

to provide mask values for 11 bit ID systems (the most

significant bit is always ignored).

Note that a common storage location is used

internally for the 29 bit and 11 bit masks, so an 11 bit

mask could conceivably be assigned with the next

command (CM hh hh hh hh), should you wish to do the

extra typing. The values are right justified, so you

would need to provide five leading zeros followed by

the three mask bytes.

CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]

This command is used to assign mask values for

29 bit ID systems. See the discussion under the

CM hhh command – it is essentially identical, except

for the length. Note that the three most significant bits

that you provide in the first digit will be ignored.

CP hh [ set CAN Priority bits to hh ]

This command is used to set the five most

significant bits in a 29 bit CAN ID word (the other 24

bits are set with the AT SH command). Some systems

use several of these bits to assign a priority value to

messages, which is how the command was named.

Any bits provided in excess of the five required will be

ignored, and not stored by the ELM327 (it only uses

the five least significant bits of this byte). The default

value for these priority bits is hex 18.

CS [ show the CAN Status ]

The CAN protocol requires that statistics be kept

regarding the number of transmit and receive errors

detected. If there should be a significant number of

them, the device can even go off-line in order not to

affect other data on the bus, should there be a

hardware or software fault. The AT CS command lets

you see both the Tx and the Rx error counts. If the

transmitter should be off (count >FF), you will see

‘OFF’ rather than a specific count.

CV dddd [ Calibrate the Voltage to dd.dd volts ]

The voltage reading that the ELM327 presents for

an AT RV reading can be calibrated with this

command. The argument (‘dddd’) must always be

provided as 4 digits, with no decimal point (it assumes

that a decimal place is between the second and the

third digits).

To use this calibration feature, simply use a meter

with sufficient accuracy to read the actual input

voltage. If, for example, the ELM327 consistently says

the voltage is 12.2V when you measure 11.99 volts,

simply issue AT CV 1199, and the device will

recalibrate itself for the provided voltage (it should then

read 12.0V due to roundoff). If you use a test voltage

that is less than 10 volts, don’t forget to add a leading

zero (that is, 9.02 volts should be entered as AT CV

0902).

D [ set all to Defaults ]

This command is used to set the options to their

default (or factory) settings, as when power is first

applied. The last stored protocol will be retrieved from

memory, and will become the current setting (possibly

closing other protocols that are active). Any settings

that the user had made for custom headers, filters, or

masks will be restored to their default values, and all

timer settings will also be restored to their defaults.

AT Commands (continued)

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ELM327

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AT Commands (continued)

DM1 [ monitor for DM1s ]

The SAE J1939 Protocol broadcasts trouble codes

periodically as they are detected, using Diagnostic

Mode 1 (DM1) messages. This command sets the

ELM327 to continually monitor for this type of

message for you, following multi-segment transport

protocols as required. Note that a combination of

masks and filters could be set to provide a similar

output, but they would not allow multiline messages to

be detected. The DM1 command adds the extra logic

needed for multiline messages.

This command is only available when a CAN

Protocol (A, B, or C) has been selected for J1939

formatting. It returns an error if attempted under any

other conditions.

DP [ Describe the current Protocol ]

The ELM327 is capable of automatically

determining the appropriate OBD protocol to use for

each vehicle that it is connected to. When the IC

connects to a vehicle, however, it returns only the data

requested, and does not report the protocol found. The

DP command is used to determine the current protocol

that the ELM327 is selected for (even if not

connected). If the automatic option is also selected,

the protocol will show the word "AUTO" before it,

followed by the type. Note that the actual protocol

names are displayed, not the numbers used by the

protocol setting commands.

DPN [ Describe the Protocol by Number ]

This command is similar to the DP command, but

it returns a number which represents the current

protocol. If the automatic search function is also

enabled, the number will be preceded with the letter

‘A’. The number is the same one that is used with the

set protocol and test protocol commands.

E0 and E1 [ Echo off (0) or on(1) ]

These commands control whether or not

characters received on the RS232 port are

retransmitted (or echoed) back to the host computer.

To reduce traffic on the RS232 bus, users may wish to

turn echoing off by issuing ATE0. The default is E1 (or

echo on).

FC SM h [ Flow Control Set Mode to h ]

This command sets how the ELM327 responds to

First Frame messages when automatic Flow Control

responses are enabled. The single digit provided can

either be ‘0’ (the default) for fully automatic responses,

‘1’ for completely user defined responses, or ‘2’ for

user defined data bytes in the response. More

complete details and examples can be found in the

Altering Flow Control Messages section.

FC SH hhh [ Flow Control Set Header to… ]

The header (or more properly ‘CAN ID’) bytes

used for CAN Flow Control response messages can

be set using this command. Only the right-most 11 bits

of those provided will be used - the most significant bit

is always removed. This command currently only

affects Flow Control mode 1.

FC SH hhhhhhhh [ Flow Control Set Header to… ]

This command is used to set the header (or ‘CAN

ID’) bits for Flow Control responses with 29 bit CAN ID

systems. Since the 8 nibbles define 32 bits, only the

right-most 29 bits of those provided will be used - the

most significant three bits are always removed. This

command currently only affects Flow Control mode 1.

FC SD [1-5 bytes] [ Flow Control Set Data to… ]

The data bytes that are sent in a CAN Flow

Control message can be set with this command. The

current version of the software allows one to five data

bytes to be defined, with the remainder of the data

bytes in the message being automatically set to the

default CAN filler byte. Data provided with this

command is only used when Flow Control modes 1 or

2 have been enabled.

H0 and H1 [ Headers off (0) or on(1) ]

These commands control whether or not the

additional (header) bytes of information are shown in

the responses from the vehicle. These are normally

not shown by the ELM327, but can be turned on by

issuing the AT H1 command.

Turning the headers on actually shows more than

just the header bytes – you will see the complete

message as transmitted, including the check-digits and

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PCI bytes, and possibly the CAN data length code

(DLC) if it has been enabled with PP 29. The current

version of this IC does not display the CAN CRC code,

nor the special J1850 IFR bytes (which some protocols

use to acknowledge receipt of a message).

I [ Identify yourself ]

Issuing this command causes the chip to identify

itself, by printing the startup product ID string (currently

“ELM327 v1.2”). Software can use this to determine

exactly which integrated circuit it is talking to, without

having to reset the IC.

IB 10 [set the ISO Baud rate to 10400 ]

This command restores the ISO 9141-2 and

ISO 14230-4 baud rates to the default value of 10400.

IB 96 [set the ISO Baud rate to 9600 ]

Several users have requested this command. It is

used to change the baud rate used for the ISO 9141-2

and ISO 14230-4 protocols (numbers 3, 4, and 5) to

9600 baud, while relaxing some of the requirements

for the initiation byte transfers. It may be useful for

experimenting with some vehicles. Normal 10,400

baud operation can be restored at any time by issuing

an IB 10 command.

IFR0, IFR1, and IFR2 [ IFR control ]

The SAE J1850 protocol allows for an In-Frame

Response (IFR) byte to be sent after each message,

usually to acknowledge the correct receipt of that

message. The ELM327 automatically generates and

sends this byte for you by default, but you can override

this behaviour with this command.

The AT IFR0 command will disable the sending of

all IFRs, no matter what the header bytes require.

AT IFR2 is the opposite - it will force an IFR byte to

always be sent, no matter what header bytes indicate.

The AT IFR1 command restores the response to

provide the automatic sending of IFRs, as determined

by the ‘K’ bit of the header byte. IFR1 is the default

setting of the ELM327.

IFR H and IFR S [ IFR from Header or Source ]

The value sent in the J1850 In-Frame Response

(IFR) byte is normally the same as the value sent as

the Source (or Tester) Address byte that was in the

header of the request. There may be occasions when

it is desireable to use some other value, however, and

this set of commands allows for this.

If you send AT IFR S, the ELM327 will use the

value defined as the Source Address (usually F1, but it

can be changed by PP 06), even if another value was

sent in the Header bytes. This is not what is normally

required, and caution should be used when using

AT IFR S. AT IFR H restores the sending of the IFR

bytes to those provided in the Header. AT IFR H is the

default setting.

IIA hh [ set the ISO Init Address to hh ]

The ISO 9141-2 and ISO 14230-4 standards state

that when beginning a session with an ECU, the

initiation sequence is to be directed to a specific

address ($33). If you wish to experiment by directing

the slow five baud sequence to another address, it is

done with this command. For example, if you prefer

that the initiation be performed with the ECU at

address $7A, then simply send:

>AT IIA 7A

and the ELM327 will use that address when called to

do so (protocols 3 or 4). The full eight bit value is used

exactly as provided – no changes are made to it (ie no

adding of parity bits, etc.)

Note that setting this value does not affect any

address values used in the header bytes, and that this

value is reset to to $33 whenever the defaults, or the

ELM327, are reset.

KW0 and KW1 [ Key Word checks off (0) or on (1) ]

The ELM327 looks for specific bytes (called Key

Words) to be sent to it during the ISO 9141-2 and

ISO14230-4 initiation sequences. If those bytes are

not found, the initiation is said to have failed (you

might see “UNABLE TO CONNECT” or perhaps “BUS

INIT: ...ERROR”). This may be because you are trying

to connect to a non-OBD compliant ECU, or perhaps

to an older one.

If you wish to experiment, but do not want the

ELM327 to check the values contained in the key

words, you can turn the checking off with:

>AT KW0

AT Commands (continued)

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