ELM327LSM datasheet(63/94 Pages) ELM

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ELM327L DSA

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ELM327L

CAN Input Frequency Matching

Most modern vehicles have a CAN network

connected to pins 6 and 14 of the OBD connector. At

one time, however, the use of these pins was left to

the vehicle manufacturer, and a number of different

systems were connected to them.

In order to prevent the disruption of any connected

systems while the ELM327 is searching for a protocol

(it sends out requests during a search), the ELM327

now performs several tests on these wires. Prior to

firmware version 2.1, the tests simply looked for

activity on the wires but were not frequency selective.

This meant that, for example, vehicles that had a

speedometer signal connected to either pin might be

seen as a valid CAN network, and the ELM327 may

have sent a request on those wires. The new firmware

actually measures the input frequency and requires

that it matches that of the selected CAN protocol

before any test message can be sent.

The diagram below shows how the logic works. It

Signal

Processing

allow a

CAN send

CAN Rx

(pin 24)

signal is 500 kbps

setting is

500 kbps

PP 28, b7 is 0

signal is 250 kbps

setting is

250 kbps

PP 28, b6 is 0

signal is not 250 or 500 kbps

setting is not

250 or 500 kbps

PP 28, b0 is 1

input is quiet

Send Logic While Searching for a Protocol

may seem a little complicated, but what it really says is

that for the default settings, a send is allowed if the

input signal frequency matches the CAN setting (250

or 500 kbps), or if there appears to be no signal. In

addition, if the user is trying a non-standard OBD

frequency, but a standard frequency is received, a

send will not be allowed.

All bits of PP 28 are set to 1 by default (requiring

frequency matching, unless no signal is detected), but

may be changed at any time – see the Programmable

Parameters section for details.

This logic is only used while searching for a valid

protocol. Once a particular protocol is considered to be

active, no further frequency checks are made (as it is

time consuming). Note that if you should use the AT BI

command to bypass the initiation process, this

frequency matching test will also bypassed.

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