Bedienungsanleitung Elm 327 (Seite 59 von 94) (Englisch)

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Periodic (Wakeup) Messages

After an ISO 9141 or ISO 14230 connection has

been established, there needs to be periodic data

transfers in order to maintain that connection, and

prevent it from ‘going to sleep.’ If normal requests and

responses are being sent, that is usually sufficient, but

the ELM327 occasionally has to create its own

messages, to prevent the connection from timing out.

We term these periodic messages that are created

the ‘Wakeup Messages’, as they keep the connection

alive, and prevent the circuitry from going back to an

idle or sleep mode. (Some texts refer to these

messages simply as ‘idle messages.’) The ELM327

automatically creates and sends these for you if there

appears to be no other activity – there is nothing that

you need do to ensure that they occur. To see that

these are being sent, simply watch the OBD transmit

LED – you will see the periodic ‘blips’ as the ELM327

sends each one.

The ELM327 normally sends wakeup messages

after 3 seconds of no activity (this time is adjustable

with the AT SW command). This is within the five

second limit set by the standard.

The default content of these messages vary

depending on the protocol - for ISO 9141, the ELM327

will send 68 6A F1 01 00, and it sends C1 33 F1 3E for

ISO 14230 (KWP). If you would prefer that a different

message be sent, simply use the Wakeup Message

command to define it.

For example, if you would like to send the data

bytes 44 55 with the header bytes set to 11 22 33,

simply send the command:

>AT WM 11 22 33 44 55

and from that point forward, every wakeup message

that the ELM327 sends will be 11 22 33 44 55 (with a

checksum byte following). You do not provide the

checksum byte - it is automatically added for you.

You can change these as often as you want, the

only restriction being that every time you do, you must

provide the complete message – the header bytes and

the data bytes. The current version of the ELM327

allows for messages of one to six bytes total, not

including the checksum.

The ELM327 does not currently support periodic

messages for the CAN protocols. If you require that

function, please consider using our ELM329 CAN

Interpreter product.

59 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM327

The NMEA 2000 Standard

We are occasionally asked about support for the

NMEA 2000 marine standard. Elm Electronics does

not provide specific support for this protocol, but our

ELM327 integrated circuit is very capable of working

with it.

While the physical connectors may look quite a bit

different than those used for J1939, the CAN interface

and the data format is almost identical to that of the

J1939 standard. NMEA 2000 uses a 250 kbps data

rate, so the easiest way to get started is to select the

ELM327’s predefined protocol A. This is done with the

set protocol to A command:

>AT SP A

When you are finished and want to use the

ELM327 for standard OBDII protocols, don’t forget to

send the AT SP 0 command to reset it.

Many of the PGNs used for NMEA 2000 have

values that are greater than 65535, so the DP bit is

usually set. To monitor for most PGNs then, you can

not use the short version of the MP command. For

example, to monitor for the Engine Parameters PGN

(127488 or hex 1F200), you can not use:

>AT MP 1F200

as the ELM327 actually interprets that as a request for

PGN 1F20, and get 0 replies. To monitor for PGN

1F200, you must send:

>AT MP 01F200

If you keep the above in mind, the ELM327 will

prove to be a handy tool to use while experimenting

with NMEA 2000. It does have a couple of limitations

that must be kept in mind, though. As mentioned with

J1939, it is not capable of address negotiation. Also,

the ELM327 does not currently support the Fast

Packet protocol, which may be an issue for some

users.

For more information on the NMEA 2000 standard,

visit the NMEA web site (http://www.nmea.org).

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