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Fault
Failure in a system that prevents normal operation of a system.
FDDI (Fibre Distributed Data Interface)
A fiber-optic communication architecture/technology providing very fast data transfer even over large distances, and inherent redundancy capabilities.
Firmware
A series of instructions contained in read-only memory (ROM) that are used for the operating system functions. Some manufacturers offer upgrades for PLCs. This is often done by replacing a ROM chip. Thus the combination of software and hardware lead to it being called firmware.
Fiber optics
A data transmission medium using light conducted through glass or plastic fibers. A fiber-optic cable has cores capable of conducting modulated light signals by internal reflection.
Fieldbus
Fieldbus is a generic term used to describe a common communications protocol for control systems and/or field instruments. Although some standard forms have been agreed for instruments, the DCS industry as a whole has so far no agreed fieldbus. Often, the Foundation Fieldbus is referred to simply as the Fieldbus.
Flash Memory
A RAM-ROM hybrid that can be erased and rewritten under software control. Such devices are divided into blocks, called sectors that are individually erasable. Flash memory is common in systems that require nonvolatile data storage at very low cost. In some cases, a large flash memory may even be used instead of a disk drive.
Force
Refers to changing the state of actual I/O by changing the bit status in the PLC. In other words, a person can force an output on by changing the bit associated with the real-world output to a 1. Forcing is normally used to troubleshoot a system.
Frame
Packet of bits that will be transmitted across a network. A frame contains a header, user data and an end of frame. The frame must contain all the necessary information to enable the sender and receiver(s) of the communication to decode the user's data and to ensure that this data is right.
Full duplex
Communication scheme where data flows in both directions simultaneous.
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