Serial communication sends data one bit at a time, with fewer cables needed than parallel communication. It’s used to connect printers, cameras, and peripherals. RS 232 connections can use up to 20 signals, but fewer signals allow for faster synchronization and reliability over long distances. Asynchronous and synchronous methods are used for synchronization, with eight data bits, one stop bit, and no parity being a common configuration. Software protocols like XMODEM or ZMODEM can be used to protect against data transmission errors.
Serial communication is a method of data transmission that sends information one bit at a time from device to device. Many different serial standards have been developed over the years for both low-speed and high-speed device bandwidths. Data can usually be exchanged over much greater distances using serial rather than parallel communication. Serial communication is usually used to connect printers, terminals and cameras to computers. It is also used to interface with external hard drives, digital video disc (DVD) drives, and flash memory devices.
Since only one bit of data is sent at a time in serial communication, fewer cables are required than with a parallel interface. A minimal connection may include only one wire for data and another for ground reference. In practice, many serial links also include several handshake signals as well as one data line in each direction. The universal serial bus (USB), commonly used to connect computers and peripherals, uses only four or five signals, two of which are for power. The recommended standard serial (RS) 232 connections can use up to 20 signals, depending on the implementation.
Fewer signals generally allow a serial communication link to synchronize faster and operate more reliably over long distances. Parallel communication can introduce distortion or interference between data bits as they travel together along a long link. RS 232 serial connections of 1,000 feet (300 meters) or more can usually be used at more than 115,200 bits per second. Conversely, USB 2.0 connections are often used to connect high bandwidth storage devices to computer systems. They can typically exchange data at up to 480 megabits per second, but cables are limited to 16 feet (5 meters) between hubs.
When data is transmitted over a serial link, the receiver must have some way of telling when each byte ends and the next begins. In asynchronous serial communication, the sender inserts a “start” bit before sending the bits of each byte. The start bit also synchronizes an internal clock to help break up the rest of the received frame into individual bits. This is the most commonly used RS 232 synchronization method. In synchronous serial communication, a separate clock signal is used to indicate when each bit and byte is complete.
Before an RS 232 data exchange begins, the devices on both sides must be set to use the same number of data and stop bits, as well as the same type of parity. Eight data bits, one stop bit, and no parity is a common configuration, commonly expressed as 8N1. If mark or space parity is used, the parity bit is correspondingly set to one or zero by the sender. If even or odd parity is used, the parity bit is set to a value that will make the total number of a bit even or odd. The receiver checks the value of the received parity bit, if present, and indicates an error if it does not match the expected value.
In addition to parity checking, one or more software serial communication protocols may be employed to protect against data transmission errors. For example, the XMODEM or ZMODEM protocol is often used for file transfers between computers over an RS 232 serial link. These protocols were originally designed for use with a dial-up telephone modem at each end of the link, but they also work without them. Each protocol includes validation of a cyclic redundancy check (CRC) calculated for the data being sent. If modems are present, they also perform similar CRC checks in hardware during transmission.
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