Program memory is the capacity for control software storage. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLC include DC, AC, relay, analog, frequency or pulse, transistor, and triac. Programming options for PLC include front panel, hand held, and computer.Programmable logic controllers can also be specifi ed with a number of computer interface options, network specifi cations, and features. PLC power options, mounting options, and environmental operating conditions are all also important to be considered.
PLCs are usually available in these three general types:
The embedded controllers expand their fi eld bus terminals and transform them into a modular PLC. All embedded controllers support the same communication standards such as Ethernet TCP/IP. The industrial embedded pc and compact operating units belonging to PLC product spectrum are also identical for all controllers.
This type of PLCs is of slide-in card for the PC that extends every PC or IPC and transforms it into a fully fledged PLC. In the PC, the slide-in card needs only one PCI bus slot and runs fully independently of the operating system. PC system crashes leave the machine control completely cold.
The compact PLC controller unites the functions of an operating unit and a PLC. To some extent, the compact controller already features integrated digital and analog inputs and outputs. Further fi eld bus terminals in the compact PLCs can be connected via an electrically isolated interface such as CANopen.
The two main factors to consider when choosing memory are the type and the amount. An application may require two types of memory: nonvolatile memory and volatile memory with a battery backup. A non volatile memory, such as EPROM, can provide a reliable, permanent storage medium once the program has been created and debugged. If the application will require on-line changes, then it should probably be stored in read/write memory supported by a battery. Some controllers offer both of these options, which can be used individually or in con junction with each other. The amount of memory required for a given application is a function of the total number of inputs and outputs to be controlled and the complexity of the control program. The complexity refers to the amount and type of arithmetic and data manipulation functions that the PLC will perform. For each of their products, manufacturers have a rule-of-thumb formula that helps to approximate the memory requirement. This formula involves multiplying the total number of I/O by a constant (usually a number between 3 and 8). If the program involves arithmetic or data manipulation, this memory approximation should be increased by 25–50%.
(c) Software considerations.
During system implementation, the user must program the PLC. Because the programming is so important, the user should be aware of the software capabilities of the product they choose. Generally, the software capability of a system is tailored to handle the control hardware that is available with the controller. However, some applications require special software functions that are beyond the control of the hardware components. For instance, an application may involve special control or data acquisition functions that require complex numerical calculations and data-handling manipulations. The instruction set selected will determine the ease with which these software tasks can be implemented. It will also directly affect the time required to implement and execute the control program.
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