Serial Cable

Information about Serial Cable

Serial Cables are typically used for RS-232 communication.

A serial cable is a cable that can be used to transfer information between two devices. Serial cables use the RS-232 standards for their connectors. The serial cable can be any combination of male or female in a DE-9 (often mistakenly called DB-9) or DB25 connector type. The original RS-232 connector was intended to be a 25-pin connector (DB25). In the realm of computer peripherals, the serial cable has been deprecated by the Universal Serial Bus (USB) standard.

History

The serial cable was often used when communicating with a computer and a peripheral (such as a modem). The term null modem is used to describe serial cables used without a modem to allow two computers to communicate directly.

Originally the serial cable was designed for a speed of 300 baud (baud rate is synonymous with symbols per second), with 1200 bit/s considered to be high speed. This is in stark contrast to early-21 century bandwidth needs, in which a common household internet connection can exceed 1,512,000 bit/s on the downstream. Later versions of the cable with supporting software and hardware could handle speeds of up to 115,200 bit/s.

Used as null modems, types:

No hardware handshaking

The most simplistic type of serial cable. This cable has only the data and signal ground wires connected. All of the other pins have no connection. With this type of cable flow control has to be implemented in the software. The use of this cable is restricted to data-traffic only on its cross connected Rx and Tx lines. This cable can also be used in devices that do not need or make use of modem control signals.

Loop back handshaking

Because of the compatibility issues and potential problems with a simple null modem cable, a solution was developed to trick the software into thinking there was handshaking available. However, the cable pin out merely loops back and does not physically support the hardware flow control.

This cable could be used with more software but it had no actual enhancements over its predecessor. The software would work thinking it had hardware flow control but could suddenly stop when higher speeds were reached and with no identifiable reason.

Partial handshaking

In this cable the flow control lines are still looped back to the device. However, they are done so in a way that still permits Request To Send (RTS) and Clear To Send (CTS) flow control but has no actual functionality. The only way the flow control signal would reach the other device is if the opposite device checked for a Carrier Detect (CD) signal (at pin 1 on a DE-9 cable and pin 8 on a DB-25 cable). As a result only specially designed software could make use of this partial handshaking. Software flow control still worked with this cable.

Full handshaking

This cable is the most expensive of the serial cables because it has full wiring and pin-outs. This cable is incompatible with the previous types of cables' hardware flow control, due to a crossing of its RTS/CTS pins. With suitable software, the cable is capable of much higher speeds than its predecessors. It also supports software flow control.

Uses

The serial cable is still in use in a few (some old or obsolete) peripheral devices. Growing use of other communication cables, such as those needed for USB and Firewire connections, have greatly declined the serial cable's application. However, the serial cable still has wide use in electronics that don't require high bandwidth communication, particularly embedded computer systems. Serial cables are also commonly used to console into a device such as a router or network switch

These devices (and others) have been known to communicate via serial cable:

Modem
Barcode scanners
Computers
Gaming devices
Scientific instruments
Medical devices
Field equipment
Military equipment
Personal digital assistant (PDA)
Printers
Uninterruptible power supply (UPS)

Maximum cable lengths

Cable length is one of the most discussed items in RS232 world. The standard has a clear answer, the maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. The latter rule is often forgotten. This means that using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. If for example UTP CAT-5 cable is used with a typical capacitance of 17 pF/ft, the maximum allowed cable length is 147 feet.

The cable length mentioned in the standard allows maximum communication speed to occur. If speed is reduced by a factor 2 or 4, the maximum length increases dramatically. Texas Instruments has done some practical experiments years ago at different baud rates to test the maximum allowed cable lengths. Keep in mind, that the RS232 standard was originally developed for 20 kbit/s. By halving the maximum communication speed, the allowed cable length increases by a factor of ten!

Baud rate	Maximum cable length (ft)
19200	50
9600	500
4800	1000
2400	3000

References

cable is one or more wires or optical fibers bound together, typically in a common protective jacket or sheath. The individual wires or fibers inside the jacket may be covered or insulated. Combination cables may contain both electrical wires and optical fibers.
..... Click the link for more information.

In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports.
..... Click the link for more information.

peripheral is a piece of computer hardware that is added to a host computer ,i.e any hardware except the computer, in order to expand its abilities. More specifically, the term is used to describe those devices that are optional in nature, as opposed to hardware that is either
..... Click the link for more information.

USB
Universal Serial Bus

Original USB Logo
Year created: January 1996

Width:
Number of devices: 127 per host
Capacity Up to 12Mbit/s (USB 1.0)
Up to 480 Mbit/s (USB 2.
..... Click the link for more information.

Modem (from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information.
..... Click the link for more information.

Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using a RS-232 serial cable. The original RS-232 standard only defined the connection of DTEs with DCEs i.e. modems.
..... Click the link for more information.

baud (pronounced /bɔːd/ unit symbol "Bd"), is a measure of the symbol rate, the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal.
..... Click the link for more information.

21st Century is the present century of the Common Era, in accordance with the Gregorian calendar. It began on January 1, 2001 and is due to end December 31, 2100. However, more modern methods of dating begin the century in the year 2000.
..... Click the link for more information.

The term downstream has several possible meanings:

In geography, downstream means literally away from the source of a stream or river, and in meteorology, away from the source of an air parcel or mass, along the normal direction of water or air flow.
..... Click the link for more information.

Not to be confused with control flow.

In computer networking, flow control is the process of managing the rate of data transmission between two nodes.
..... Click the link for more information.

USB
Universal Serial Bus

Original USB Logo
Year created: January 1996

Width:
Number of devices: 127 per host
Capacity Up to 12Mbit/s (USB 1.0)
Up to 480 Mbit/s (USB 2.
..... Click the link for more information.

FireWire

Year created: 1990
Created by: Apple

Width:
Number of devices: 63
Capacity 400/800 Mbit/s
Style: Serial
Hotplugging? Yes
External? Yes

FireWire is Apple Inc.
..... Click the link for more information.

router is a device that extracts the destination of a packet it receives, selects the best path to that destination, and forwards data packets to the next device along this path.^[1] They connect networks together; a LAN to a WAN for example, to access the Internet.
..... Click the link for more information.

barcode (also bar code) is a machine-readable representation of information (usually dark ink on a light background to create high and low reflectance which is converted to 1s and 0s).
..... Click the link for more information.

Personal digital assistants (PDAs) are handheld computers, but have become much more versatile over the years. PDAs are also known as pocket computers or palmtop computers.
..... Click the link for more information.

uninterruptible power supply (UPS), also known as an uninterruptible power source or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not
..... Click the link for more information.

A LapLink cable (also known as lablink or null-printer cable) is a cable that allows one to connect two computers together to establish a direct cable connection. The connection is achieved via the parallel ports on the two computers.
..... Click the link for more information.

Direct Cable Connection (DCC), is a feature of Microsoft Windows that allows a computer to transfer and share files (or connected printers) with another computer, via a connection using either the serial port, parallel port or the infrared port of each computer.
..... Click the link for more information.

Client-server is a computing architecture which separates a client from a server, and is almost always implemented over a computer network. Each client or server connected to a network can also be referred to as a node.
..... Click the link for more information.

Wi-Fi (pronounced wye-fye, IPA: /ˈwaɪfaɪ/), also unofficially known as Wireless Fidelity
..... Click the link for more information.

Bluetooth is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices such as mobile phones, laptops, PCs, printers, digital cameras, and video game consoles over a secure, globally
..... Click the link for more information.

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer, however a bridge works by using bridging where traffic from one
..... Click the link for more information.

In telecommunications and computer science, serial communications is the process of sending data one bit at one time, sequentially, over a communications channel or computer bus. This is in contrast to parallel communications, where all the bits of each symbol are sent together.
..... Click the link for more information.

A legacy port is a port or connector on a PC that is considered fully or partially obsolete. Some manufacturers, particularly in laptops, remove the legacy ports in lieu of "modern" ports such as USB ports.
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.

Herod_Archelaus

iPedia Free Information Center