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rs232 serial port

Serial Communication

Serial Communication

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Serial communication is the famous and broadly used technique to transfer data between processing equipment and peripherals. Serial communication is used to transfer data across the WANs. The sending router encapsulates the data using a communication protocol. The encapsulated data is sent on a physical medium to the WAN and receiving router de-encapsulates the encapsulated data back.

Serial communication uses Binary One for representing logic HIGH or 5 Volts and binary zero for representing a logic LOW or 0 Volts. It can take many forms depending on the type of transmission mode and data transfer. The transmission modes are Simplex, Half Duplex, Full Duplex, Synchronous and Asynchronous. The source is known as a sender and the destination is known is a receiver for each transmission mode.

Some well-known interfaces used for the data exchange are RS-232, RS-422, RS-423, V.35, HSSI, RS-485, etc. Each one is using a different signalling method. The three most important serial communication standards are affecting LAN-to-WAN connections. We should discuss these most important methods.

Asynchronous Data Transfer – In the Asynchronous Data Transfer the bits of data are not synchronized by a clock pulse. The clock pulse is a signal used for synchronization.

Synchronous Data Transfer – In the synchronous mode the bits of data are synchronized using a clock pulse.

RS-232

RS-232 is referring to the Interface between data terminal equipment (DTE) and data communications equipment (DCE) using serial binary data exchange. The data terminal equipment (DTE) is usually the user computer, while data communications equipment (DCE) is the modem. RS-232 was introduced in the 1960s and originally known as EIA recommended standard 232. It is the oldest serial communication standard.

Most of the serial ports on personal computers are RS-232C or newer standards RS-422 and RS-423. All the standards use both 9-pin and 25-pin connectors. Modems, mice, keyboards and printers were uses serial port but now these types of peripheral devices have been replaced by new and faster standards such as USB and RJ-45 connectors.

RS-232 are commonly available with 4, 9 or 25-pin wiring. The 25-pin Rs232 known as DB-25 connects every pin while the 9-pin Rs232 known as DB-9 does not connect many of the uncommonly used connections. The 4-pin Rs232 cables provide the minimum connections for data transferring.

Most RS-232 devices can operate with a minimum of 3 signal cables: Transmit (TX), Receive (RX) and Ground (GND). The TX cable of one device must be connected with the RX cable of other device and vice versa and the ground cable must be connected.

A 25 pin RS-232 port on a PC transmits on pin 2 and receives on pin 3, and Ground is pin 7. In the 9-pin RS-232 port on a PC transmits on pin 3, receives on pin 2 and Ground is pin 5. We cannot connect simply two devices with a serial cable because the connectors fit. It is important to verify the functions of each pin on each device, as well as verify whether or not the cable is a straight-through or null-modem cable.

V.35

The V.35 interface was originally specified by CCITT as an interface for 48kbps line transmissions. It has been adopted for all line speeds above 20kbps and seems to have acquired a life of its own. It was discontinued by CCITT in 1988, and replaced by recommendations V.10 and V.11.

It is an International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) specification standardized as an interface for 48 kbps line transmissions. The standard is used for modem-to-multiplexer communication. The standard is used for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. The standard provides high data transfer rates (DTR) and connectivity between DCE and DTE over digital lines. In the U.S., this is used by most routers and DSUs that connect to T1 carriers.

The V.35 interface is working on layer 1 of the Open Systems Interconnection (OSI) model. V.35 interface is usually used for 56 kbps and 64 kbps data rates. It has balanced data and clock leads including handshake leads. The V.35 has a 34 pin connector that combines the bandwidth of several telephone lines. The combined telephone lines provide a high-speed interface between DTE and channel service units. Both balanced and unbalanced voltage signals on the same interface provide better speed and distance. We can travel the V.35 cable up to 1200 meters with 100 Kbps speed.

The V.35 plug is standard with black plastic cover about 20mm by 70mm size, often with gold-plated contacts and built-in hold down and mating screws. The V.35 plug is 20 to 40 times the price of a DB25. Financially the RS-232 option is economical than the V.35 option.

HSSI

HSSI is the abbreviation of High-Speed Serial Interface, which is used for short-distance communications. Usually, HSSI is used to interconnect routing and switching devices on local area networks (LANs) with the higher-speed lines of a wide area network (WAN) such as T3 lines. It is a serial interface that supports data transmission up to 52 Mbps. HSSI is developed by Cisco system for DTE and DCE. HSSI can transmit data up to a distance of 52 feet. HSSI is working at the physical layer of the OSI model. HSSI is used as a 50-pin connector. It is used ECL technology for transmission.

Serial Port

Serial Port

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A serial port allows a PC or network devices to transmit or receive data one bit at a time. A serial interface is the oldest types of an interface that was also used to connect printers and external modems to a PC at that time. Modern serial ports are used in scientific instruments such as cash registers and applications like industrial machinery systems. The data transfer rate is very slow then the parallel port.

The serial port is used for point-to-point connection such as LAN-to-WAN connection. The LAN-to-WAN connection over a serial interface is also known as leased line connection because the line is usually leased from the telephone companies and the subscribers pay for those lines to hold the continuous connection between two sites.

Usually serial port is a male port and system resource configurations are chosen for each port and are recognized by COM1, COM2, COM3, COM4, and so forth. Each COM position represents an input/output (I/O) and an interrupt request (IRQ) address. The I/O address sends and receive packets to and from a peripheral device such as a mouse or keyboard or any network devices.

Using the serial connection on the network, we can send and receive data bits sequentially over a single channel. The serial port is bidirectional, and usually known as a bidirectional port or a communications port.

A serial connection was primarily used for signal exchange because of their bidirectional ability. Serial communications are less costly to implement because it is used fewer wires, cheaper cables, and fewer connector pins. On most computers, parallel ports and serial ports have been replaced by the higher speed serial Universal Serial Bus (USB) interfaces. Though, for long-distance communication, many WANs still uses serial transmission. This standard used for serial transmission between devices is RS 232 and usually called data communications equipment (DCE) and data terminal equipment (DTE). The serial port uses a nine-pin connector which is known as DB-9 connector.

Today the Higher speed communication port Universal Serial Bus (USB) and FireWire introduced faster interfaces; therefore, the serial port is hardly ever used but can be found as a communication device for Radios, GPS receivers, LED and LCD text displays, bar-code scanners and flat-screen monitors.

Serial Port DB-9 Connector

The figure below illustrates the serial connector DB-9. The figure shows each of the pins located on the DB9 connector. The Pin 1 starts from the top left and pin 9 is in the bottom right.

Serial Port

The serial ports carry data signals and control signals. To support these signal types including ground signals, RS-232 standard defines a 25-pin connection known as a parallel port. However, most PC’s and UNIX platforms use a 9-pin connection. Actually, only three pins are needed for serial port communications: pin for sending data, pin for receiving data, and pin for the signal ground. The table below illustrates the signals associated with the DB-9 pin connectors.

The RTS and CTS Pins

The RTS and CTS pins transmit signals that the devices are ready to send or receive data. It is providing hardware handshaking and control data loss during transmission. The TD pin is used to send data between DCE and DTE.

The DTR and DSR Pins

These Pins are used for showing that the device is connected and powered. These pins provide an alternative method of hardware handshaking.

The CD and RI Pins

The CD and RI pins are used to show different signals during modem to modem connections. The modem gets indication from CD pin that it is connected to another modem. RI is used to indicate the presence of an audible ringing signal.