Campbell Scientific SDM-CAN Datalogger to CANbus Interface - Manual (Page 24)

SDM-CAN Datalogger-to-CANbus Interface
range of baud rates. However, be sure to check that these are correct for your
specific network before using them.
The parameters are entered as integer numbers which define various times that
control when the binary data is sampled by the CAN hardware. The following
discussion and nomenclature is common to the set-up of most CAN controller
chips. If you are not familiar with CAN at this level please seek the advice of
someone who is familiar with your network to determine these parameters.
The overall speed of the network is specified by the baud rate, in bits per
seconds, which define the time per bit (
t
bit
) by the simple relationship:
t
bit
= 1 / baudrate
Within the time period for each bit the CAN standards define three different
time segments which ultimately control when the CAN hardware samples the
signal.
This is often shown in a diagram, thus:
S-SG PROP_SEG PHASE_SEG1 PHASE_SEG2
Bit time (t
bit
)
Sample point
1 time-quanta (t
q
)
t
TSEG1
t
TSEG2
The bit time is divided into time-quanta ( t
q
), of which there are between 8-23
time- quantum in the bit time. The t
q
(in seconds) used by the SDM-CAN is
set by the scaling factor TQUANTA (parameter 02). This is the parameter that
largely determines the baud rate. To work out a suitable value of TQUANTA,
knowing the required
t
q
, the following equation is used:
TQUANTA = t
q
* 8*10 6
The first time segment is known as the synchronization segment (S-SG) and
by convention is one time-quanta long.
This is followed by two segments known as the propagation segment and phase
segment one. These are determined by the characteristics of the network and
other devices on the network. The total of these two time segments determines
the time
when the SDM-CAN samples the data bit and is known as t
TSEG1
. The final
segment is known as phase segment two or t
TSEG2
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