Measurement Computing DBK70 - Manual (Page 73)

DBK70 User’s Manual 928494 Fundamentals of Obtaining Vehicle Data 6-7
The J2190 document shows byte #1 as the mode number, but in actuality, there is an implied 3-byte header
on each message that contains the Type & Priority, Target address, and Source address bytes.
The Type & Priority byte is defined by the manufacturer of the vehicle, the Target is the address of the
network module holding the desired data, and the Source is the DBK70. For this example, configure the
DBK70’s channel 1 with a Message field of C410F1221165. Since the target module will only respond
when this message is sent, set the channel’s Update Rate to the desired sample frequency.
In the previous diagram, the J2190’s byte #4 description directs the reader to the definition of “Data byte #3
for Mode $21”. Mode 21 describes this byte as follows.
Byte Value Description
No byte 1 response
00 Stop sending data
01 Send 1 response
02 Repeat at slow rate
03 Repeat at medium rate
04 Repeat at fast rate
05-FF Manufacturer defined
To capture the response, the channel’s Filter must match the unique features of the response message
broadcast by the target network module. The diagram below shows how the 2
nd part of the J2190 document,
“Vehicle Response to Request for Data”, is used to develop the DBK70’s Filter.
For the response, the DBK70 is now the Target of the network message and the power train module,
10H, is the Source. The mode is 62H, as defined in the standard (typically, the mode for the response
message is equal to the mode of the request message plus 40H). After the Filter is developed, the mask
bytes must be applied so that the DBK70 will know which bits are significant and which are not. In many
cases, only a small part of the return message is truly needed for matching. For example, if RPM is desired,
the value of the Type & Priority byte is probably irrelevant, and it likely doesn’t matter who it is directed to
(the target) or who sent it (the source). In this case, only the mode and PID bytes are significant, all other
bytes could be masked with 00H, or don’t care.