BR16F84-1.07 ETC, BR16F84-1.07 Datasheet - Page 7

• Manufacturer Part Number: BR16F84-1.07
• Description: OBD II Interface Chip
• Manufacturer: ETC

SAE (VPW or PWM) COMMUNICATIONS
For these protocols, the chip can only buffer one frame of data, so you must specify which frame it should
grab and return. The vehicle may send more than one frame, so if it’s a data item that can possibly have a
multiple frame response, you would need to repeat the request until you have obtained all of the frames.
Generally, this is only necessary for a few types of data.
The Request message always takes the following form:
[Control byte], [SAE Request Message], [Frame number]
As usual, the Control byte just indicates the total number of bytes to follow. The SAE request message is
defined in SAE J1850 and J1979. It is composed of three header bytes, a series of message bytes, and a
CRC byte. Note that the SAE Request Message conforms strictly to the relevant SAE specifications, while
the control byte and the Frame number are required by the chip. The SAE messages are outside the scope
of this data sheet but are available from any number of sources.
The Response message always takes the following form if the resu lt was a success;
[Control byte], [SAE Response Message]. The control byte simply indicates how many bytes follow. These
bytes are the SAE response message, and they contain the header bytes, frame message bytes for this
response, and the CRC byte as per S AE specs referenced above.
The Response message always takes the following form if the result was not successful;
[Control byte], [Status byte]. The control byte will have the most significant bit set. The lower 4 bits will be
001, which simply indicates that one byte follows, and it is the status byte for internal conditions. This is a
commonly occurring condition, because specifications allow a vehicle to return no data, or invalid data if the
requested information is not supported by the vehicle, or if the data is not currently available in the vehicle's
processor. If the chip received no response, or if it received an invalid response, the most significant bit of
the control byte will be set, and one byte of internal condition status will follow.
ISO 9141-2 COMMUNICATIONS
The ISO 9141-2 system used by many Asian and European vehicles is quite different from SAE vehicles, so
it is discussed separately. The request message from the host is very similar to SAE vehicles, but the chip
does not require or allow a frame number. The request message is simply the control byte indicating the
number of bytes following, then the request message frame, including the checksum. The response from the
chip is simply a pure retransmission of data received from the vehicle. There is no control byte, so the host
should simply receive bytes until a time-out period of 55 Milliseconds occurs with no bytes received. This
indicates the end of the vehicles response. The response is therefore one or more message frames as
defined in SAE J1979. The chip performs no analysis of frames, does not reject non-diagnostic frames, etc.
The host must process this data to extract particular frames by looking for the header bytes. For most data
requests, there is only one response frame.
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