Question

 

From the given list below of networking standards used in PLC systems, evaluate each,

Peer To Peer

ISO

 

EVALUATE each of the given networking signal protocols used in PLC systems

RS232

IE488

20mA

Answer 1

Peer to Peer: Peer-to-peer networks enhance reliability by decentralizing the control functions without sacrificing coordinated control. In this type of network, numerous PLCs are connected to one another in a daisy-chain fashion, and a common memory table is duplicated in the memory of each. In this way, when any PLC writes data to this memory area, the information is automatically transferred to all other PLCs in the network. They then can use this information in their own operating programs. With peer-to-peer networks, each PLC in the network is responsible for its own control site and only needs to be programmed for its own area of responsibility. In a peer-to-peer system, there's no master PLC. However, it's possible to designate one of the PLCs as a master for use as a type of group controller. This PLC then can be used to accept input information from an operator input terminal, for example, sending all the necessary parameters to other PLCs and coordinating the sequencing of various events.

Remote I/0 systems :-A remote I/O configuration has the actual inputs and outputs at some distance from the controller and CPU. This type of system, which can be described as a "master-and-slave" configuration, allows many distant digital and analog points to be controlled by a single PLC. Typically, remote I/Os are connected to the CPU via twisted pair or fiber optic cables. Remote I/O configurations can be extremely cost-effective control solutions where only a few I/O points are needed in widely separated areas. In this situation, it's not always necessary, or practical for that matter, to have a controller at each site. Nor is it practical to individually hard wire each I/O point over long distances back to the CPU. For example, remote I/O systems can be used in acquiring data from remote plant or facility locations. Information such as cycle times, counts, duration or events, etc. then can be sent back to the PLC for maintenance and management reporting.

RS232:- This is an IEEE standard for serial communications where one bit of data is sent at a time along a single data line that describes specific wiring connections, voltage levels, and other operating parameters for electronic data communications. RS232 defines the signals connecting between DTE and DCE. Here, DTE stands for Data Terminal Equipment and an example for DTE is a computer or PLC. DCE stands for Data Communication Equipment or Data Circuit Terminating Equipment and an example for DCE is a modem.

IE488:- IE 488 is a digital communications bus specification invented by Hewlett Packard and used to connect short range communication devices. This term is also known as the general purpose interface bus (GPIB) or the Hewlett Packard interface bus (HP-IB). The IE 488 has a 24-pin connector and is used for double headed design. Both ends of the cable are used, male on one side and female on other side. The IE 488 has 16 signal lines. Eight lines are dedicated for bi-directional communication, five lines are used for bus management. The remaining three lines are dedicated for handshakes. This allows 15 devices to be shared over a single physical bus.

20mA:- The 20 mA current loop scheme is has been used for many years for sending digital data. Although not a formal standard, it is a de-facto standard that that was widely used for many serial data communications applications. A major application of current loops is the industry standard 4–20 mA current loop for process control applications, where they are extensively used to carry signals from process instrumentation to PID controllers, SCADA systems, and programmable logic controllers (PLCs). They are also used to transmit controller outputs to the modulating field devices such as control valves. These loops have the advantages of simplicity and noise immunity, and have a large international user and equipment supplier base. Some 4–20 mA field devices can be powered by the current loop itself, removing the need for separate power supplies.