Campbell Scientific SDM-SIO1 Serial Input/Output Module - Manual (Page 7)

SDM-SIO1 Serial Input/Output Module
The SDM-SIO1 Module is designed to allow expansion of the number of serial ports
available on a datalogger for communicating with intelligent sensors or driving external
displays.
1. Introduction
The SDM-SIO1 Module connects to Campbell Scientific dataloggers using the
SDM port and communication protocol. It connects to the remote serial device
using industry standard hardware that can be set to RS-232, RS-485 or RS-422
signal levels. When operating in RS-232 mode it also supports hardware
handshaking. RS-422 mode is functionally the same as RS-485 mode except
the connection is limited to a point to point system. Connections and
programming for RS-422 are otherwise identical to RS-485.
The SDM-SIO1 will accept serial data and store it in its buffer which is 2047
bytes in size allowing remote equipment to transmit large amounts of data
without needing to stop other processes in the datalogger. Up to 15 SDM-SIO1
peripherals can be connected to a single logger using the SDM port, allowing
the user to connect 15 different items of equipment to their logger with ease, in
addition to any connections made to the datalogger’s other serial ports.
To start using the SDM-SIO1 it is first necessary to work out how data will be
exchanged with a sensor. In the case of a sensor there are basically two options,
either the datalogger requests data and then picks up the response, or the sensor
transmits data “one-way” using its own time base. The latter mode is more
common but can lead to problems with synchronizing the sensor measurements
with the logger program and can also lead to the occasional missing data value
as there are two independent clocks. Once the method of communication and
the communication standard is defined then refer to Section 3, Installation, to
install the module and connect it to the datalogger and the serial device.
The SDM-SIO1 Module is implemented in such a way that it looks like a built-
in serial port to the user when writing programs in CRBasic. This means all the
user needs to do is define the address of the SDM-SIO1, using a rotary switch
on the side of the unit. The serial port can then be used as if it were built into
the datalogger. The only difference in operation between the SDM-SIO1 and a
built-in port is that there will be a small delay when transferring data to and
from the device via the SDM connection (see Section 4.1, Special Information
about the Serial I/O() CRBasic Instructions).
The SDM-SIO1 can also be used in ‘talk-through’ mode to allow a user to talk,
via a terminal module, to a sensor connected to the SDM-SIO1 for test and
diagnostic purposes. Please refer to the logger manual for further details.
Section 4, Programming the Datalogger, gives the differences in the use of this
module compared to the datalogger standard serial ports, plus there are some
simple examples.
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