Read n Write

The language of PLCs consists of a commonly used set of terms; many of which are unique to PLCs. In order to understand the ideas and concepts of PLCs, an understanding of these terms in necessary. Sensor A sensor is a device that converts a physical condition into an electrical signal for use by the PLC. Sensors are connected to the input of a PLC. A pushbutton is one example of a sensor that is conected to the PLC input. An electical signal is sent from the pushbutton ton the PLC indicating the condition (open/closed) of the pushbutton contacts. Actuators Actuators convert an electrical signal from the PLC into a physical condition. Actuators are connected to the PLC output. A motor starter is one example of an actuator that is conected to the PLC output. Depending on the output PLC signal the motor starter will either start or stop the motor. Discrete Input A discrete input, also referred to as a digital input, is an input that is either in an ON or OFF conditio...

Programmbable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLCs monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine. This course is meant to supply you with basic information on the functions and configurations of PLCs. Basic PLC Operation   PLCs consist of input modules or points, a Central Processing Unit (CPU), and output modules or points. An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and excecutes control instructions based on program instructions in memory. Output modules convert control instructions from the CPU into a digital  or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired ins...

Most SCADA half-duplex protocols do not allow one slave station to talk to another slave station, exept throught special application-soecific  code, which requires processing overhead in the master station. However, Allan-Bradleys DF1 half -duplex protocol implements slave-to-slave communications as a feature of the protocol within the master station, wirhout any additional application code or extra procesing overhead. Refer to chapter 3 of the DF1 Protocol and Commond Set Reference Manual, pulication 1770-RM516, for additional information. If one slave station has a message to send to another, it simply includes the destination slave station's address in the message instrucrion's destination field in place of the master station's address when responding to a poll. The master starion checks the destination station address in every packet header it receives from any slave station. If the address does nt match its own station address, the entire message is forwarded back on...

From the introduction of actuators and transducers (that made monitoring of processes easier, more accurate and less costly) at the instrumentation level to the introduction of open standards (to improve the interchange of data between a SCADA sysem and other processes within an organisation), SCADA systems have exploited the various technological advances to drive forward their proficiency. The drive of modern SCADA systems is to: Provide instrumentation and Rus/PLCs for asset or process solutions that can be easily managed and to provide operational benefits from the SCADA host down to the instumentation, not just in terms of controlling and retriecing data but also engineering, implementing, operating and maintaining these assets. Develo and employ open standards to duther ease the integration of assets within a SCADA system using best practices defined by open groups and not a single manufacturing entily. This will in turn reduce the cosr of owning SCADA. Provide secure envir...

 Security for SCADA systems has in recent years become animportant and hotly debated topic. Traditionally SCADA systems were isolated entities that were the relm of operators enginners and technicians. Ths has meant that SCADA Host platforms were not necessarily developed to have protected connections to public networks. This left many SCADA host platforms open to attack as they did not have the toos necessary to protect themselves. In terms of remote assets communicating back to a SCADA Host, security has been an issue for many years with numerous documented attacks on SCADA systems. However, it's only been in recent years that an open standard has been produced to provide secure encrypted and authenticated data exchanges between remote assets and a SCADA Host platform. Conclusion of SCADA   About Slave-to- Slave Messaging SCADA Programmable Logic Controllers (PLCs) Terminology of PLCs   The L3GD203-Axis Gyro Solutions for remote asset and SCADA host communic...

Traditionally, SCADA Host software has been the mechanism to view graphical displays alarms and trends. Control from the SCADA Host itself only became available when control elements for remote instruments were developed. These systems were isolated fro the outside world and were the domain of operaors, technicians and engineers. Their responsibility was to monitor, maintain and engineer processes and SCADA elements. With advancements in Ingormation Tecnology (IT) this is no longer the case. Many different stake holdes now require real time access to the data that the SCADA Host software generates. Accounting, maintenance management and material purchaasing requirements are preformed or party preformed from data derived from the SCADA system. SCADA Systems SCADA Field INstrumentation PLCs and RTUs Remote Communications Networks Consequently, there is a drive for the SCADA Host to be an Enterprise entity providing data to a number of different users and processes. This has encora...

The remote commuication network is necessary to relay data from remote RTU/PPLCs, which are out in the field or along the pipeline, to the SCADA host located at the field office or central control center. With assets distributed over a large geographical area, communicaton is the glue or the linking part of a SCADA system and essential to its operation. How well a SCADA system can manage communication to remote assets is fundamental to how sucessful the SCADA system is. Twnty years ago the comunication network would have been leased lines or dial-up modems which were very expenive to install and maintain, but in the last 10-15 years many users have switched to radio or satellite communications to reduce costs and eliminate the problematic cabling issues. More recenly, other communication tyes have been made available that include cellular communictions and improved radio devices that can support greater communication rates and better diagnosics. However, the fact hat these types of c...