Table of Contents
Advertisement
SDM-SIO1A and SDM-SIO4A Serial Input/Output Modules
6.1
Mounting
8
account for the differences in wiring a sensor to the SDM-SIO1A rather than
directly to the datalogger.
For other sensors or more complex applications, a CRBasic program needs to
be written to configure and enable the connection between the two devices.
To write a program, it is first necessary to determine how data will be
exchanged. When using the SDM-SIO1A to collect data from a sensor, there
are two options. The datalogger requests data and then picks up the data in the
response from the sensor (often called polling) or the sensor transmits data
"one-way" to the datalogger at fixed intervals based upon its internal timer.
The latter mode is more common but can lead to problems with synchronizing
the sensor measurements with other measurements the datalogger is making
and can also lead to the occasional missed data value if the sensor clock is
running slightly slower than the datalogger clock.
The SDM-SIO1A is implemented in such a way that it looks like a built-in
serial port to the user when writing programs in CRBasic. The only difference
in operation between the SDM-SIO1A and a built-in port is that there will be a
small delay as the data needs to be transferred from the SDM-SIO1A to the
datalogger via the SDM port. That delay is usually only significant if large
amounts of data are being read or the datalogger program needs to run very
fast.
For sending and receiving data, the SerialOpen() instruction is used to specify
which serial port, the speed, and mode of operation. The speed is set by the
BaudRate parameter in the instruction and the mode (bits, parity, duplex, RS-
232 or RS-485) is set by the SerialOpenFormat parameter. Refer to the
CRBasic Editor help system for more details of the options. With the correct
parameters in the SerialOpen() instruction, the program can be written in
almost exactly the same way as using any other serial port with SerialIn(),
SerialOut(), and similar instructions. Section 6.3, Programming
manual gives details of some of the minor changes needed and some simple
program examples.
The SDM-SIO1A can also be used as an interface to Modbus and DNP3
sensors or networks. The interface is configured by including an extra
SerialOpen() instruction at the start of the program. Normal calls to the
relevant Modbus or DNP3 instructions can then be used specifying the
COM port number relevant to the SDM-SIO1A.
The SDM-SIO1A is normally mounted on the backplate of a Campbell
Scientific enclosure using the screws and plastic inserts provided. The
SDM-SIO1A is designed to be installed in a dry, non-condensing environment.
Before mounting, select and set the SDM address as this requires access to the
side of the case. The SDM address is set with a screwdriver. TABLE
the possible SDM addresses and their relationships to the COMPort parameter
in the SerialOpen() instruction.
There can be up to 15 SDM-SIO1As on a single SDM bus. Each SDM-SIO1A
will need to be set to a unique address before they are powered. If any other
equipment is present on the bus, whether an SDM-SIO1A or other SDM
device, the user will have to ensure their addresses are unique.
, of this
(p. 16)
6-1
lists
Advertisement
Table of Contents
