Campbell Scientific SDM-CAN Datalogger to CANbus Interface - Manual (Page 21)

SDM-CAN Datalogger-to-CANbus Interface
CAN errors detected and to also determine the general status of the SDM-CAN
interface.
3.2 System Limitations
The SDM-CAN interface, in combination with a datalogger, has some
limitations of which you need to be aware:
1) Memory Allocation and P118
Firstly, as discussed above, when the datalogger compiles a program with
P118 in it, it sends commands to the SDM-CAN instructing it what to do
at run time. When it does this the SDM-CAN allocates some of its
memory (a ‘bin’) for each call of P118 in the program. Appendix A,
Principles of Operation, discusses the operation of these bins and other
buffers in the SDM-CAN in more detail. However, most users only need
to know that there is a limit of 128 bins in the SDM-CAN thus
constraining the number of instances of P118 for any one SDM-CAN to
128.
It is, of course, possible to have several SDM-CAN devices connected to
the datalogger(s), each with separate SDM addresses, and each with up to
128 calls of P118.
2) Data Capture Limitations
Another limitation is the capability of the overall speed at which the
datalogger can pick up and transfer data values back to its memory. These
limitations do not arise within the SDM-CAN interface itself, as it uses a
high speed CAN interface along with a fast microprocessor. Data can
therefore be captured off the CAN-Bus at close to the maximum bus
loading at the maximum baud rate. However, the limitations arise from the
datalogger itself, both in terms of its capability to call P118 often enough
(especially when making other measurements) and also in its capability to
transfer the data from the SDM-CAN back into its memory over the SDM
communications port.
The exact throughput possible is determined by a very complicated
combination of variables, including the speed of the datalogger in
question, the program it is running, how many SDM devices are in use
and, to a lesser degree, other tasks it is running, e.g. communications
activity.
In practice, for fast data, it will not be practical to capture every single data
packet. However, the SDM-CAN will be used to sample the last reading it
received on the CAN-Bus before the datalogger requests data.
If a new data value has not been captured from the CAN-Bus since the last
value was transferred to the datalogger, the SDM-CAN can either be set to
always return the previous value captured (default) or it can be configured
(see the internal software switch settings below) to return the standard out
of range value to the datalogger, i.e. –99999 if the value has already been
read. This value will also be returned in the event of other errors including
communication errors between the datalogger and SDM-CAN.
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