Measurement Computing DBK70 - Manual (Page 105)

Appendix B 937696 Scale & Offset in Summary View B-1
Appendix B – Scale and Offset in Summary View B
Note: This appendix is for users of Summary View who want to understand the mathematics behind
scale and offset functions.
One of the most important concepts to understand when configuring a DBK70 output channel is the
concept of Scaled Received Data (SRD). The value of output channel signals are based on its SRDs.
SRD is the result of scaling data from a received message.
A DBK70 output channel configuration includes information that identifies:
• The received message that will include the data to be processed for the channel
• Where [in the received message] the data begins (Data Field Start), what the size of the data
is (Data Field Length), and whether the data is to be interpreted as a signed or as an
unsigned value (Data Field Type)
• The scale and offset parameters (Output Scale and Output Offset) that are to be used to
calculate the SRD
The SRD that results from this process is the input to the process that determines the signal generated by
the output channel.
The output channel configuration parameters mentioned above, the receive message filtering process, the
scaling process, the output signal generating process, and other associated processes that result in
producing SRD and the output signal will be described in further detail below. Where SRD is referenced in
this document, it is the result of the scaling process applied to data received from a vehicle data bus
message and the value used to determine the characteristics of the signal generated by an output channel.
Another scaling process that uses a separate set of scale and offset parameters (i.e., PC Display Scale and
PC Display Offset) included in a channel configuration is used to scale the received data for display in an
Output Channel Icon on a PC screen. The same received message data used to calculate the SRD is used to
calculate the data value displayed in the Output Channel Icon. These parameters and the data display
process will be described in further detail below. The scaling process used here is the same process used to
calculate the SRD.
Mathematically, the SRD is interpreted as a 32 bit signed integer value (i.e., a long integer in C). The SRD
is the result of multiplying the received data by the Output Scale and adding to the result of the
multiplication the Output Offset and then converting the result this addition into an 32 bit signed integer
value (i.e., y = mx + b, where SRD = integer of the y value, where x is the received data, m is the scale
value, b is the offset value, and y = the result of the scaling process).
The received data is the data from a message that was received from a vehicle data bus and passed through
the Filter defined for an output channel. The Index and Length specify where in the received message the
data starts and how long it is. Storage Type specifies whether the received data is interpreted as either a
signed or an unsigned value. The Output Scale and Output Offset values may be positive or negative and
may be defined in an integer or floating point form (i.e., floating point means defined with a decimal point
and with or without fractional values). An SRD may be a positive or a negative value.
The signal generated by an output channel is determined by the SRD’s value and the way a given output
channel processes that value. Storage Type, Output Mode, and Aux Value determine the way an output
channel processes an SRD.