Filter
Top Products
SDM-CAN CAN-Bus Interface User Guide
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 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 practise, 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.
Data stored in packets on the CAN-Bus can be encoded in a number of
different ways. The SDM-CAN itself can cater for many different types of
data, but there are some limitations imposed by the way in which the data is
stored in the datalogger. The prime limitation is that data read into the
datalogger is first converted into a 4 byte floating point format which can only
resolve, at most, 23 bits, or roughly 7 digits, of the decimal equivalent of any
number stored. Furthermore, when data is stored to final storage, the
resolution is truncated again to either 4 or 5 digits (with the exception of the
CR5000/9000 dataloggers which also support storage in IEEE4 format).
To avoid over-running the datalogger’s internal floating point resolution, the
maximum length of integer that the SDM-CAN can send or receive is
therefore limited to 16 bits. This limited resolution can cause problems when
reading CAN data where data is encoded as 32 or 64 bit integers.
3-2