Campbell Scientific SDM-CAN Datalogger to CANbus Interface - Manual (Page 35)

SDM-CAN Datalogger-to-CANbus Interface
following the end of one interrupt before the SDM-CAN will raise the port for
another interrupt. This could be a limitation in high speed data capture
applications, hence the need for switch 2.
When switch 2 is set, the SDM-CAN responds immediately to data receipt and
raises the port as soon as data has been received, filtered and processed. The
SDM-CAN will only lower the line again permanently when the datalogger
reads the data out of the SDM-CAN that caused the interrupt. To prevent
problems with some events which might cause the datalogger to miss
interrupts, the SDM-CAN will pulse the I/O port low for 1 ms after 50 ms, take
the line high and then repeat this cycle until all the relevant data has been read.
Using this switch setting will provide the quickest way of capturing data but
may not work with other devices sharing the datalogger interrupt port.
To ensure proper configuration of the SDM-CAN by the
datalogger for interrupt driven applications, it will pulse its I/O
port on and off at 50ms intervals for 6 seconds after power-up or
program recompilation. NOTE
3.4.2 Group Trigger
The group trigger function provides a mechanism to synchronize the data
capture by one or more SDM-CAN (and some other SDM devices too).
This mode is enabled when an SDM-Group Trigger (P110) instruction is
encountered. When this instruction runs, it broadcasts a special SDM message
which causes all the SDM-CAN devices to copy the last data values captured
from the CAN-bus into the working data buffers, and no further updates are
allowed until P110 runs again (normally at the next execution of the program
table). P118 instructions will read the locked values which are all sampled at
once.
This SDM-Group trigger command is normally positioned at the beginning of
the program table to lock all data samples exactly to the start of the scan
interval. It should be remembered, however, that in the case of the SDM-CAN
it will simply lock these values to the last values captured which could already
have been transmitted some time earlier.
The SDM-Group trigger instruction actually broadcasts its message to SDM
address 33
4
(base 4), which prevents this address being available if the SDM-
Group trigger command is to be used. This effectively reduces the number of
SDM peripherals that support global trigger to 15 units.
3.4.3 Frame buffers with filtering and triggering
Operating systems V3 include the ability for the user data logger program to
attach a buffer of 256 frames to any receiving CAN ID up to a limit of 25
different ID’s.
If the user program tries to allocate more than 25 buffers then the
additional buffer allocations will be ignored. NOTE
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