ELM634
Error Messages
There are only a few error messages that the
ELM634 will generate in response to data problems.
They are kept short so that their sending does not
interfere with the LIN data monitoring. Here is a brief
description of each:
[S]
[ Sync error ]
If the ELM634 is unable to synchronize to the LIN
data stream, or if the received data bytes seem to
be of incorrect length, a synchronizing error occurs.
This might be for several reasons – if the LIN bus
rate is something other than 9600bps, if the data
stream is not of the LIN format and sync breaks or
bytes could not be found, or possibly if something
occurred during data transfer and the stop bits did
not occur when expected. The ELM634 will report
this error, and immediately try to re-synchronize to
the bus (and will continue to do this indefinitely, or at
least until interrupted by the user).
[T]
[ Timeout error ]
This message is printed if there have been no
recognizable messages for at least 25000 bit times
(referred to as a ‘Bus Idle Time-Out’ in the LIN
specification). This timeout normally occurs when
the LIN system has been put to sleep.
When a timeout occurs, the ELM634 will continue to
monitor the bus, and will resume resending the LIN
data when it appears again.
?
[ General error ]
This is the standard response for a general error,
often due to a misunderstood AT command received
on the RS232 bus (usually due to a typing mistake).
It can also occur if the checksum byte that was
received was not correct for either a classic (LIN1.3)
or an enhanced (LIN2.0) system.
Example Applications
Figures 2, 3 and 4 on the following pages show
various ways that the ELM634 might typically be
connected to monitor a LIN system. They show a full
implementation; a less expensive, but just as capable
one; and finally, a minimal circuit that can essentially
be built into a cable to automatically translate LIN data
to ASCII RS232 data.
The LIN standard does not specify what type of
connector is to be used, but it does specify that the
physical interface will essentially be an enhanced
implementation of the ISO 9141 standard currently
used in automobiles. It provides a three wire
connection, with power, data, and common. You will
have to choose a connector (or clips) as appropriate
for your application.
Figure 2 on the following page is a complete
implementation, which uses a standard RS232
ELM634DSB
interface IC to simplify the connections to the RS232
bus. This is necessary because the ELM634 logic
levels are not compatible with RS232 levels. Note that
pin 4 must be set to a logic high level in this case to
disable the internal inverter. One way of remembering
the correct level to set pin 4 to is to consider the logic
level that pin 5 (RS232Rx) normally rests at, and set
pin 4 to that same level. The RS232 pin connections
shown are for a 25 pin female connector, as would
often be found on an external modem. If you are using
a 9 pin connector to match a cable from your
computer, the connections should be changed to
2(RxD), 5(SG) and 3(TxD).
Since the LIN standard provides for a (nominal)
12V supply at the connector, that convenient source
has been used to power the ELM634 circuit. This
voltage is reduced to the required 5 volts by the 78L05
8 of 11
Elm Electronics – Circuits for the Hobbyist
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