Serial Cables

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. more...

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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 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.

Used as null modems, types:

No 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. As a result only specially designed software could make use of this partial handshaking. Software flow control still worked with this cable.

Read more at Wikipedia.org