Operation Manual

62 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
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.