BR16F84-1.07 ETC, BR16F84-1.07 Datasheet - Page 6

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

INITIALIZING THE CHIP AND THE VEHICLE
Before normal communications can start, the host must establish communication with the chip, then it must
initialize the chip and the vehicle's data link.
After the unit is connected to the host computer and the vehicle, an initialization sequence must be done.
This sequence is used to prevent hangups due to noise on the serial lines i n case they have not been
connected before the chip is powered up. It also serves as a simple method to detect whether the interface is
connected or not. The first operation is to send a single byte, 20H. This is a connect command to the chip,
and it tells the chip that communication is to commence. The chip does not send a control byte in return, but
responds with a single byte which is FF Hex, or decimal 255. At this point, the chip is waiting to receive
data, and the host may then proceed with initialization. NOTE that this is one of the few cases where the
chip does not send a control byte.
This initializes the protocol to be used, and also initializes the vehicle in the case of ISO. There are three
protocols, VPW (general motors), PWM (Ford products), and ISO 9141-2 (Asian/European). There are many
exceptions to these general protocols, for example, some Mazda's use Ford's PWM protocol. So, if you have
problems with a particular protocol, you should try another. The protocol selection is done by sending a
control byte which is 41H, followed by a protocol select byte. The latter byte is defined as follows; 0=VPW,
1=PWM, 2=ISO 9141. For example, 41H 02H is the sequence to initialize to ISO 9141 protocol.
The chip will respond with a control byte and a status byte. The control byte will have the MSB set if there
was a problem, and in this case the next byte is a status byte which indicates internal conditions. If the
initialization was a success, the control byte will be 01H, which indicates one verification status byte follows.
This status byte is a verification byte and is defined as follows; For VPW or PWM, this byte is simply an
echo of the protocol byte that you sent (0 or 1). For ISO 9141 it will be a "KEY number" that was returned
from the vehicle itself and specifies one of two slightly different versions of ISO. It is for informational
purposes only. Note that for VPW and PWM, the protocol select is rapid, because it is only needed to inform
the chip which protocol is to be used. But, for ISO vehicles, it can take up to approximately 5 seconds,
because there is a detailed initialization sequence that the chip must perform at 5 baud with the vehicle. Also
NOTE that some legacy ISO 9141 vehicles will lose initialization if no data requests are made during a 5
second interval, so the host PC should automatically send a request every few seconds durin g idle periods.
Once connection and initialization is complete, messages would typically be request messages from the host
computer, or response messages from the chip.
REQUEST AND RESPONSE MESSAGES
The operation of this chip is significantly different for the SAE (VPW or PWM) protocols, and the legacy
ISO9141-2 protocols, so the information is presented separately for the two cases to avoid confusion.
(1) CHIP CONNECT
(2) INITIALIZATION
5