<Li> No method is specified for sending power to a device . While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low - power devices such as mice . </Li> <Li> The 25 - pin D - sub connector recommended in the standard is large compared to current practice . </Li> <P> In the book PC 97 Hardware Design Guide, Microsoft deprecated support for the RS - 232 compatible serial port of the original IBM PC design . Today, RS - 232 has mostly been replaced in personal computers by USB for local communications . Advantages compared to RS - 232 are that USB is faster, uses lower voltages, and has connectors that are simpler to connect and use . Disadvantages of USB compared to RS - 232 are that USB is far less immune to electromagnetic interference (EMI) and that maximum cable length is much shorter (15 meters for RS - 232 v.s. 3 - 5 meters for USB depending on USB speed used). </P> <P> In fields such as laboratory automation or surveying, RS - 232 devices may continue to be used . Some types of programmable logic controllers, variable - frequency drives, servo drives, and computerized numerical control equipment are programmable via RS - 232 . Computer manufacturers have responded to this demand by re-introducing the DE-9M connector on their computers or by making adapters available . </P>

What is the maximum length of a standard rs-232 cable