Introduction - Campbell SDM-SIO1A Instruction Manual

SDM-SIO1A Serial Input/Output Module
The SDM-SIO1A 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. The SDM-SIO1A is a functional replacement for the SDM-SIO1 interface, being
slightly smaller and having different labelling for some terminals (see Section 3.1.1).
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Introduction

SDM-SIO1A Serial Input/Output Module
The SDM-SIO1A Module connects to Campbell Scientific dataloggers using the
SDM port and communications 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. RS422 mode is functionally the same as RS485 mode except the
connection is limited to a point to point system. Connections and programming for
RS422 are otherwise identical to RS485.
The SDM-SIO1A will accept serial data and store it in its buffer, which is 6143
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-SIO1As 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 dataloggers other serial ports.
To start using the SDM-SIO1A 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 timebase. The latter mode is more
common but can lead to problems with synchronising 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
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