Serial cable

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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 DE9 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.

1 History
2 Used as null modems, types:
3 Uses
4 Maximum cable lengths
5 See also
6 References

[edit] 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 bits 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 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.

[edit] Used as null modems, types:

[edit] 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.

[edit] 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.

[edit] 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. As a result only specially designed software could make use of this partial handshaking. Software flow control still worked with this cable.

[edit] 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 this cable and the suitable software to take advantage of it, the cable is capable of much higher speeds than its predecessors. It also supports software flow control.

[edit] Uses

The serial cable is still in use in a few (some old or obsolete) peripheral devices. Growing use of USB technology, since the 1990s, has greatly declined the serial cable's application.

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)

[edit] 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 kbps. By halving the maximum communication speed, the allowed cable length increases a factor ten!