Bedienungsanleitung Elm 327 (Seite 75 von 94) (Englisch)

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Maximum CAN Data Rates (continued)

processed. You can see from the figure that even for a

500 kbps message with an 11 bit ID, the ELM327

finishes with time to spare. Since ISO15765-4

specifies that messages must be 8 data bytes in length

(filler bytes are added as needed) these times do

represent the typical situation, with a 40% bus load.

Actually, from these numbers the ELM327 should be

able to handle 100% bus loading (which is not a

practical situation).

Once the ELM327 has placed all of the properly

formatted bytes into the RS232 transmit buffer, it is up

to the controlling computer to fetch them in a timely

fashion. If the bytes are removed too slowly, the buffer

will continue to fill as new OBD messages arrive, and

the buffer will eventually become full. It does not

matter how big the buffer is, if the rate of removing

bytes from the buffer is slower than the rate of putting

them into the buffer, it will eventually fill up. When it is

full, you will see a ‘BUFFER FULL’ message, and you

will have to start over.

The ELM327 transmit buffer is 512 bytes in size.

Considering that some bytes will be sent while new

messages are being queued, this means that you can

typically store:

38.4k 115.2k 500k

11 bit/500k 28 38 –

29 bit/250k 26 56 –

messages in the buffer, if the bus loading is at 40%,

as shown. This storage is more than enough for

almost all OBD requests – the only time that you might

get into trouble is if you are monitoring all messages

on the bus (AT MA) with no filters set. In that case, you

would need to be sure that you are removing bytes as

fast as they are being generated.

The rate at which OBD messages occur depends

on the ‘bus loading’. This is a utilization factor that is

very similar to the duty cycle for a square wave signal.

Ideally, bus loading should be less than about 30%,

but as vehicles become more complex, this is very

difficult to do. Some vehicles are reportedly seeing

70% bus loads.

The above chart does not show any numbers in

the 500 k column, as the buffer should never fill up

when bus loading is 40%. As you increase loading,

you will eventually reach the limit, but even with very

busy data busses, we typically see about 150

messages before a BUFFER FULL is reported. If we

turn off spaces and Linefeeds (AT S0, AT L0), we

usually do not see any BUFFER FULL errors, even

with very busy busses.

When people ask us then, ‘What data rate can the

ELM327 support?’ the answer is not easy to provide,

as it depends on many factors. It depends on the CAN

message content, the data rate, and whether you have

selected filters to eliminate irrelevant messages. The

rate also depends on the RS232 baud rate that you

have chosen, as it may be the limiting factor if there

are a great number of messages being retrieved. If

you are only fetching 10 or 20 messages, however, the

internal buffer takes care of them for you, and the

baud rate that you choose does not matter.

If you are simply fetching PID responses from the

network, there really is no limitation within the

ELM327, and it does not matter what the ‘maximum

CAN data rate’ is. If you are trying to ‘push the

envelope’, monitoring everything that travels on a very

busy CAN data bus, then there will be limitations.

What they are depend to a great deal on what choices

you make. Hopefully this discussion will have helped

to give you the necessary background information to

do so.

75 of 94

ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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