Campbell Scientific SDM-IO16 16-Channel Input/Output Expansion Module - Manual (Page 15)

SDM-IO16 16 Channel Input/Output Expansion Module
Each SDM-IO16 needs to be set to a unique address on the SDM bus, so that
no two SDM peripherals share the same address.
6. Programming the Datalogger
6.1 General Principles
For recent dataloggers, there is a specific program instruction that is used to
control operation of the SDM-IO16. The different variants of this command
are described in Section 6.2.1 and Section 6.3.1 below. Older dataloggers can
use different commands to control the output functions only; if you have such
a datalogger please go to Section 6.3.2 now as the remainder of this section
will not be relevant.
The general form of the command specific to the SDM-IO16 includes:
• parameters to specify which module to address,
• a command code to specify what the module is to do, and
• a number of control parameters and pointers to input locations /variables
which can hold data (either read by the module or which are used to
control the outputs of the module).
The instruction differs from many others in that, rather than addressing a
variable number of ports (using a 'reps' parameter), ports are dealt with in
blocks of 1, 4, 8 or 16 ports at a time. The specific port(s) and number to be
controlled is implicit in the command code used. This approach has been taken
to improve the efficiency of programming and also to reduce the amount of
data transferred between the module and datalogger via the SDM port.
The module can be reconfigured quite extensively from the controlling
program to allow for more demanding applications. In most cases, though,
operation is quite simple as on power-up the input/output ports of the SDM-
IO16 default to input mode, with no switch debounce filtering and
measurement of frequency and duty-cycle automatically starts for all ports.
For many input measurements, the controlling program in the datalogger can
then simply be written to ask the SDM-IO16 for measurements of status, duty-
cycle or frequency from any channel. The only constraint is that it must ask for
duty cycle or frequency measurements more frequently than once every
15.9375 seconds (see Appendix A).
Where one or more channels will be used for output, one call of the instruction
can be within the normal program structure to set those ports to either a fixed
state, or a state dependent upon a variable in an input location.
Where ports will be used as switch closure inputs, where a change of state of a
port will be used to toggle the I/O line (normally to generate an interrupt to the
datalogger) or where the port is to be used as a fixed output, then it would be
usual to include code that sends a command to the SDM-IO16 to configure the
ports before they are used. See the program examples in Section 7.
The configuration of individual ports can also be changed during normal
program operation, if required.
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