ELM327LP datasheet(75/94 Pages) ELM

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ELM327L

ELM327L DSA

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

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 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

ELM327LP Datasheet(PDF) 75 Page - ELM Electronics

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