6. Installation
The SDM-SIO2R is an addressable interface; therefore, before use, its SDM address needs to be
set by rotating the Address switch using a screwdriver. If there is only one SDM device, the
address can be left at the default of 0. (Refer to
[p. 9] if there is more than one SDM
device.) After mounting as close to the data logger as possible (to minimize the cable runs), the
interface is connected to the data logger SDM and power connections. Refer to
(p. 13) for further details.
Study the documentation of the serial device you want to connect to the SDM-SIO2R to
determine the most appropriate method of connection for the application, for example, RS-232
or RS-485/RS-422, half or full duplex. Generally, RS-485/RS-422 connections are used for cable
runs over 50 m (164 ft) if the device supports them. Once decided, connect the wires to the
relevant SDM-SIO2R terminals. Make sure a connection is made between the ground
connections of the two devices. Use the RG terminal on the SDM-SIO2R for RS-485/422
connections.
Some serial sensors are supported by the Campbell Scientific
Short Cut
program. The
SDM-SIO2R can be used with those programs simply by selecting the relevant serial port number
to match the SDM-SIO2R (defined by the SDM address of the SDM-SIO2R, see
Care must be taken when interpreting any wiring instructions generated by
Short Cut
to account
for the differences in wiring a sensor to the SDM-SIO2R rather than directly to the data logger.
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-SIO2R to collect data from a sensor, there are two options. The data logger 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 data logger 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 data logger is making and can also lead to the
occasional missed data value if the sensor clock is running slightly slower than the data logger
clock.
The SDM-SIO2R 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-SIO2R and
a built-in port is that there will be a small delay as the data needs to be transferred from the
SDM-SIO2R to the data logger via the SDM port. That delay is usually only significant if large
amounts of data are being read or the data logger program needs to run very fast.
SDM-SIO2R Serial Input/Output Module with Switched Sensor Power
8
