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Polling, or interrogation, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output (I/O), and is also referred to as polled I/O or software-driven I/O. A good example of hardware implementation is a watchdog timer.
Polling is the process where the computer or controlling device waits for an external device to check for its readiness or state, often with low-level hardware. For example, when a printer is connected via a parallel port, the computer waits until the printer has received the next character. These processes can be as minute as only reading one bit. This is sometimes used synonymously with 'busy-wait' polling. In this situation, when an I/O operation is required, the computer does nothing other than check the status of the I/O device until it is ready, at which point the device is accessed. In other words, the computer waits until the device is ready. Polling also refers to the situation where a device is repeatedly checked for readiness, and if it is not, the computer returns to a different task. Although not as wasteful of CPU cycles as busy waiting, this is generally not as efficient as the alternative to polling, interrupt-driven I/O.
In a simple single-purpose system, even busy-wait is perfectly appropriate if no action is possible until the I/O access, but more often than not this was traditionally a consequence of simple hardware or non-multitasking operating systems.
Polling is often intimately involved with very low-level hardware. For example, polling a parallel printer port to check whether it is ready for another character involves examining as little as one bit of a byte. That bit represents, at the time of reading, whether a single wire in the printer cable is at low or high voltage. The I/O instruction that reads this byte directly transfers the voltage state of eight real world wires to the eight circuits (flip flops) that make up one byte of a CPU register.
Polling has the disadvantage that if there are too many devices to check, the time required to poll them can exceed the time available to service the I/O device.
Polling can be described in the following steps:
Host actions:
Controller actions:
A polling cycle is the time in which each element is monitored once. The optimal polling cycle will vary according to several factors, including the desired speed of response and the overhead (e.g., processor time and bandwidth) of the polling.
In roll call polling, the polling device or process queries each element on a list in a fixed sequence. Because it waits for a response from each element, a timing mechanism is necessary to prevent lock-ups caused by non-responding elements. Roll call polling can be inefficient if the overhead for the polling messages is high, there are numerous elements to be polled in each polling cycle and only a few elements are active.
In hub polling, also referred to as token polling, each element polls the next element in some fixed sequence. This continues until the first element is reached, at which time the polling cycle starts all over again.
Polling can be employed in various computing contexts in order to control the execution or transmission sequence of the elements involved. For example, in multitasking operating systems, polling can be used to allocate processor time and other resources to the various competing processes.
In networks, polling is used to determine which nodes want to access the network. It is also used by routing protocols to retrieve routing information, as is the case with EGP (exterior gateway protocol).
An alternative to polling is the use of interrupts, which are signals generated by devices or processes to indicate that they need attention, want to communicate, etc. Although polling can be very simple, in many situations (e.g., multitasking operating systems) it is more efficient to use interrupts because it can reduce processor usage and/or bandwidth consumption.
A poll message is a control-acknowledgment message.
In a multidrop line arrangement (a central computer and different terminals in which the terminals share a single communication line to and from the computer), the system uses a master/slave polling arrangement whereby the central computer sends message (called polling message) to a specific terminal on the outgoing line. All terminals listen to the outgoing line, but only the terminal that is polled replies by sending any information that it has ready for transmission on the incoming line. [1]
In star networks, which, in its simplest form, consists of one central switch, hub, or computer that acts as a conduit to transmit messages, polling is not required to avoid chaos on the lines, but it is often used to allow the master to acquire input in an orderly fashion. These poll messages differ from those of the multidrop lines case because there are no site addresses needed, and each terminal only receives those polls that are directed to it. [1]
The IBM 3270 is a family of block oriented display and printer computer terminals introduced by IBM in 1971 and normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text color on the original models, these terminals are informally known as green screen terminals. Unlike a character-oriented terminal, the 3270 minimizes the number of I/O interrupts required by transferring large blocks of data known as data streams, and uses a high speed proprietary communications interface, using coaxial cable.
In digital computers, an interrupt is a request for the processor to interrupt currently executing code, so that the event can be processed in a timely manner. If the request is accepted, the processor will suspend its current activities, save its state, and execute a function called an interrupt handler to deal with the event. This interruption is often temporary, allowing the software to resume normal activities after the interrupt handler finishes, although the interrupt could instead indicate a fatal error.
Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format that is independent of any given processor's native bus. Devices connected to the PCI bus appear to a bus master to be connected directly to its own bus and are assigned addresses in the processor's address space. It is a parallel bus, synchronous to a single bus clock. Attached devices can take either the form of an integrated circuit fitted onto the motherboard or an expansion card that fits into a slot. The PCI Local Bus was first implemented in IBM PC compatibles, where it displaced the combination of several slow Industry Standard Architecture (ISA) slots and one fast VESA Local Bus (VLB) slot as the bus configuration. It has subsequently been adopted for other computer types. Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as Universal Serial Bus (USB) or serial, TV tuner cards and hard disk drive host adapters. PCI video cards replaced ISA and VLB cards until rising bandwidth needs outgrew the abilities of PCI. The preferred interface for video cards then became Accelerated Graphics Port (AGP), a superset of PCI, before giving way to PCI Express.
The PDP-8 is a family of 12-bit minicomputers that was produced by Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's lifetime. Its basic design follows the pioneering LINC but has a smaller instruction set, which is an expanded version of the PDP-5 instruction set. Similar machines from DEC are the PDP-12 which is a modernized version of the PDP-8 and LINC concepts, and the PDP-14 industrial controller system.
Direct memory access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).
A Universal Asynchronous Receiver-Transmitter is a protocol for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel. The electric signaling levels are handled by a driver circuit external to the UART. Common signal levels are RS-232, RS-485, and raw TTL for short debugging links. Early teletypewriters used current loops.
In computing, a parallel port is a type of interface found on early computers for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once, as opposed to serial communication, in which bits are sent one at a time. To do this, parallel ports require multiple data lines in their cables and port connectors and tend to be larger than contemporary serial ports, which only require one data line.
I2C (Inter-Integrated Circuit; pronounced as “eye-squared-C”), alternatively known as I2C or IIC, is a synchronous, multi-master/multi-slave (controller/target), single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is widely used for attaching lower-speed peripheral ICs to processors and microcontrollers in short-distance, intra-board communication.
Apple Desktop Bus (ADB) is a proprietary bit-serial peripheral bus connecting low-speed devices to computers. It was introduced on the Apple IIGS in 1986 as a way to support low-cost devices like keyboards and mice, allowing them to be connected together in a daisy chain without the need for hubs or other devices. Apple Desktop Bus was quickly introduced on later Macintosh models, on later models of NeXT computers, and saw some other third-party use as well. Like the similar PS/2 connector used in many PC-compatibles at the time, Apple Desktop Bus was rapidly replaced by USB as that system became popular in the late 1990s; the last external Apple Desktop Bus port on an Apple product was in 1999, though it remained as an internal-only bus on some Mac models into the 2000s.
PIC is a family of microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division. The name PIC initially referred to Peripheral Interface Controller, and is currently expanded as Programmable Intelligent Computer. The first parts of the family were available in 1976; by 2013 the company had shipped more than twelve billion individual parts, used in a wide variety of embedded systems.
Serial Peripheral Interface (SPI) is a de facto standard for synchronous serial communication, used primarily in embedded systems for short-distance wired communication between integrated circuits.
In computer science, asynchronous I/O is a form of input/output processing that permits other processing to continue before the I/O operation has finished. A name used for asynchronous I/O in the Windows API is overlapped I/O.
The Low Pin Count (LPC) bus is a computer bus used on IBM-compatible personal computers to connect low-bandwidth devices to the CPU, such as the BIOS ROM, "legacy" I/O devices, and Trusted Platform Module (TPM). "Legacy" I/O devices usually include serial and parallel ports, PS/2 keyboard, PS/2 mouse, and floppy disk controller.
In computing, channel I/O is a high-performance input/output (I/O) architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. In the past, channels were generally implemented with custom devices, variously named channel, I/O processor, I/O controller, I/O synchronizer, or DMA controller.
The PS/2 port is a 6-pin mini-DIN connector used for connecting keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older DE-9 RS-232 "serial mouse" connector, while the PS/2 keyboard connector replaced the larger 5-pin/180° DIN connector used in the IBM PC/AT design. The PS/2 keyboard port is electrically and logically identical to the IBM AT keyboard port, differing only in the type of electrical connector used. The PS/2 platform introduced a second port with the same design as the keyboard port for use to connect a mouse; thus the PS/2-style keyboard and mouse interfaces are electrically similar and employ the same communication protocol. However, unlike the otherwise similar Apple Desktop Bus connector used by Apple, a given system's keyboard and mouse port may not be interchangeable since the two devices use different sets of commands and the device drivers generally are hard-coded to communicate with each device at the address of the port that is conventionally assigned to that device.
The IBM 3705 Communications Controller is a simple computer which attaches to an IBM System/360 or System/370. Its purpose is to connect communication lines to the mainframe channel. It was a first communications controller of the popular IBM 37xx series. It was announced in March 1972. Designed for semiconductor memory which was not ready at the time of announcement, the 3705-I had to use 1.2 microsecond core storage; the later 3705-II uses 1.0 microsecond SRAM. Solid Logic Technology components, similar to those in S/370, were used.
Intel 8237 is a direct memory access (DMA) controller, a part of the MCS 85 microprocessor family. It enables data transfer between memory and the I/O with reduced load on the system's main processor by providing the memory with control signals and memory address information during the DMA transfer.
The Serial Input/Output system, universally known as SIO, was a proprietary peripheral bus and related software protocol stacks used on the Atari 8-bit family to provide most input/output duties for those computers. Unlike most I/O systems of the era, such as RS-232, SIO included a lightweight protocol that allowed multiple devices to be attached to a single daisy-chained port that supported dozens of devices. It also supported plug-and-play operations. SIO's designer, Joe Decuir, credits his work on the system as the basis of USB.
I3C is a specification to enable communication between computer chips by defining the electrical connection between the chips and signaling patterns to be used. Short for "Improved Inter Integrated Circuit", the standard defines the electrical connection between the chips to be a two wire, shared (multidrop), serial data bus, one wire (SCL) being used as a clock to define the sampling times, the other wire (SDA) being used as a data line whose voltage can be sampled. The standard defines a signalling protocol in which multiple chips can control communication and thereby act as the bus controller.
The NEC μCOM series is a series of microprocessors and microcontrollers manufactured by NEC in the 1970s and 1980s. The initial entries in the series were custom-designed 4 and 16-bit designs, but later models in the series were mostly based on the Intel 8080 and Zilog Z80 8-bit designs, and later, the Intel 8086 16-bit design. Most of the line was replaced in 1984 by the NEC V20, an Intel 8088 clone.