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Elm 327 Manual

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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
… 75 …


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