ELM322P View Datasheet(PDF) - Elm Electronics

Part Name

Description

Manufacturer

ELM322P

OBD (VPW) to RS232 Interpreter

Elm Electronics 

ELM322

AT Commands (continued)

I

[ Identify yourself ]

Issuing this command causes the chip to identify

itself, by printing the startup product ID string (this is

currently ‘ELM322 v2.0’). Software can use this to

determine exactly which integrated circuit it is talking

to, without resorting to resetting the entire IC.

L0 and L1

[ Linefeeds off (0) or on (1) ]

Whether or not the ELM322 transmits a linefeed

character after each carriage return character is

controlled by this option. If an ATL1 is issued,

linefeed generation will be turned on, and for ATL0, it

will be off. Users may wish to have this option on if

using a terminal program, but off if using a custom

interface (as the extra characters transmitted will

only serve to slow the vehicle polling down). The

default setting is L1 (linefeeds on).

MA

[ Monitor All messages ]

Using this command places the ELM322 into a bus

monitoring mode, in which it displays all messages

as it sees them on the OBD bus. This continues

indefinitely until stopped by activity on the RS232

input. To stop the monitoring, one should send any

single character then wait for the ELM322 to respond

with a prompt character (‘>’). Waiting for the prompt

is necessary as the response time is unpredictable,

varying depending on what the IC was doing when

interrupted. If for instance it is in the middle of

printing a line, it will first complete the line then

return to the command state, issuing the prompt

character. If it was simply waiting for input, it would

return immediately. The character which stops the

monitoring will always be discarded, and will not

affect subsequent commands.

the third byte in the message). As with the MA and

MR monitoring modes, any RS232 activity (single

character) will abort the monitoring.

PD

[ send Packed Data ]

This option is for those who are building a computer

interface and want the fastest data transfer rate

possible while still operating at 9600 baud. When

selected, responses from the vehicle will be

formatted as an initial length byte followed by the

actual response bytes from the vehicle, with no

trailing carriage returns or linefeed characters. The

data will not be altered in any way, except for the

conversion to standard RS232 bytes.

Note that the length byte only represents the total

number of data bytes following, and does not include

itself. Also, if there was a data (checksum) error, the

length byte will have its most significant bit set, so

the user should always check first to see if the length

is greater than 127. (The other 7 bits still provide a

valid byte count if there is an error, so one need only

ignore the msb, or subtract 128 from the value.)

A ‘NO DATA’ response has no data bytes, but still

sends a length byte with value ‘0’.

R0 and R1

[ Responses off (0) or on (1) ]

These commands control the ELM322’s automatic

display of responses. If responses have been turned

off, the IC will not wait for anything to be returned

from the vehicle after sending a request, and will

return immediately to waiting for RS232 commands.

This is useful if sending commands blindly when

using the IC for a non-OBD network application, or

simulating an ECU in a basic learning environment.

The default is R1 (responses on).

MR hh

[ Monitor for Receiver hh ]

This command also places the IC in a bus monitoring

mode, displaying only messages that were sent to

the hex address given by hh (messages which are

found to have that value in their second byte). Any

RS232 activity (single character) aborts the

monitoring, as with the MA command.

MT hh

[ Monitor for Transmitter hh ]

Another monitoring command, MT hh displays only

messages sent by Transmitter address hh (given by

SH xx yy zz

[ Set the Header to xx yy zz ]

This command allows the user to control the values

that are sent as the three header bytes in the

message. The value of hex digits xx will be used for

the first or priority/type byte, yy will be used for the

second or target byte, and zz will be used for the

third or source byte. These remain in effect until set

again, or until restored to the default values with the

AT D, or AT Z commands. The default header values

are 68 6A F1, as required by the SAE J1979

Diagnostic Test Modes (OBDII) standard.

ELM322DSF

Elm Electronics – Circuits for the Hobbyist

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