Operation Manual

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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
We are occasionally asked what the maximum
data rate is that the ELM327 can handle. This is often
after someone has tried to monitor all data using the
default settings and has received a ‘BUFFER FULL’
error. It is difficult to say exactly what the maximum
rate is, however, as several factors are involved.
The CAN ‘engine’ inside the ELM327 is actually
configured with one receive register that accepts
messages from the data bus, and another register that
accepts messages from the first. As long as the
firmware empties the second register before the first
register needs it, there should not be any overflow
problems with this component. The ELM327 actually
moves the data quickly to temporary storage, so this is
never a problem.
It would be nice if all the firmware had to do was to
empty the second register, and wait for it to fill again,
but that is not so. It must also check for errors,
possibly queue a CAN response, format the received
Maximum CAN Data Rates
CAN Data
message message
message from ECU
11 bit/500 = 220
29 bit/250 = 520
min space
11 bit/500 = 16
29 bit/250 = 32
At 40% Bus Loading:
11 bit/500 = 354
29 bit/250 = 828
Notes:
- all times are in µsec
- times shown are averaged typical
- messages have 8 data bytes, and:
headers on (AT H1),
spaces off (AT S0),
linefeeds off (AT L0)
Moving Data
Processing Data
Sending Results
Rx register is empty and can
accept the next message
moves and error checks
500k = 29
250k = 41
all data is in the
send buffer
formatting & preparation
11 bit ID = 193
29 bit ID = 210
send time depends on message length and baud rate,
but on average is:
38.4k 115.2k 500k
11 bit ID 5180 1740 400
29 bit ID 6475 2175 500
this is a background task - the ELM327 can
do other things while it is sending RS232
message, convert it to ascii, load it into the RS232
transmit buffer, and then prepare to receive the next
message. These tasks can take a considerable time,
depending on what formatting options you have
chosen, and the baud rate that you select.
The diagram below shows these processes
grouped into blocks. The times shown are typical for
an ISO15765-4 message, and as you can see, vary
with both the length of the CAN message and the CAN
baud rate. All values shown are time measurements in
µsec (microseconds).
When a message arrives, the ELM327 moves
quickly to move the received bytes from the special
CAN registers, so that they do not affect the next
message that arrives. The data is then formatted (as
ASCII bytes) and placed into the RS232 transmit
buffer, for sending to the controlling processor. As long
as messages do not arrive at a rate that is faster than
the ELM327 can process them in, all messages will be