User Manual

www.moxa.com [e-mail]59
Cellular Networks
3.6 How to Connect I/O Devices to Cellular Networks
SCADA Meets Ethernet
SCADA (Supervisory Control and Data Acquisition) is a computer-based industrial control system that plays an
important role in the field of automation today. In particular, SCADA systems are used to monitor and control
a process, including manufacturing, production, power generation, fabrication, refining, water treatment,
distribution, wastewater collection and treatment, oil and gas pipelines, electrical power transmission and
distribution, facility processes, and more. Some of the benefits provided by SCADA include:
• Setting up Communications for Data Acquisitions
• Graphics HMI
• Alarms
• Trends and Process Analyst
• Commands and Controls
SCADA usually refers to centralized systems that monitor and control entire sites, or a network of systems
spread out over large areas. Most control actions are performed automatically by intelligent I/O devices (i.e.,
Moxa’s Active Ethernet I/O), remote terminal units (RTUs), or programmable logic controllers (PLCs). Host
control functions are usually restricted to basic overriding or supervisory level intervention. For example, a PLC
may control the flow of cooling water through part of an industrial process, but the SCADA system may allow
operators to change the setpoints for the flow, and enable alarm conditions, such as loss of flow and high
temperature, to be displayed and recorded. The feedback control loop passes through the Active Ethernet I/O
or RTU or PLC, while the SCADA system monitors the overall performance of the loop.
Modem Extension Mode (Virtual Modem)
Data acquisition begins at the RTU, PLC, or I/O device level, and includes meter readings and equipment
status reports that are communicated to the SCADA system as required. Data is then compiled and
formatted in such a way that a control room operator using the HMI can make supervisory decisions to
adjust or override normal I/O controls. An HMI is usually linked to the SCADA system’s database and
software programs, to provide trending, diagnostic data, and management information such as scheduled
maintenance procedures, logistic information, detailed schematics for a particular sensor or machine, and
expert-system troubleshooting guides.
Data may also be fed to a commodity database to allow trending and other analytical auditing. SCADA
systems typically implement a distributed database, commonly referred to as a tag database, which
contains data elements called tags or points. A point represents a single input or output value monitored
or controlled by the system. Points can be either “hard” or “soft”. A hard point represents an actual input
or output within the system, while a soft point results from logic and math operations applied to other
points. (Most implementations conceptually remove the distinction by making every property a “soft”
point expression, which may, in the simplest case, equal a single hard point.) Points are normally stored as
value-timestamp pairs (a value and the timestamp when it was recorded or calculated). A series of value-
timestamp pairs gives the history of that point. It’s also common to store additional metadata with tags,
such as the path to a field device or PLC register, design time comments, and alarm information.