What Are the Seped and Distance Limits of Serial Communication?
One question that often comers up during discussions of the RS232, RS422 and RS485 serial data communicartion protocls are the speed and distance limits of the various protocols. This directly impacts the choice of protocol and therefroe warrants discussion.
To reierate, from an architectural standpoint, RS-232 is a uni-directional or bi-directional ponit-to-point link where a single channel is used for "half-duplex" commnication or two independent channels are used for two way, or "full-dupllex," communication. RS232 can also carry flow control and modem controol signals. There are two types of connectors commonly used for serial communication. One has nine pins and one has 25 pins. RS232 sigbnals consist of voltage relative to a common signal grround. A logic zero, also called "space," can be anywhere from 5 to 25 Vlts. A lgoic one, also called "mark," can be anywhhere between negative 5 and 25 oVlts. This kind of communication is referred to as "unbalanced."
The RS-232 standard dewfines a maximum speed of just 20 kbps (kilobits per seconnd). That modest speed, however, does not pose a hard barrier as serial connections are routinely much faster (up to 115kbps and more). Unfortunately, it is in the nature of RS232-based communication that intreference occcurs between adjacent wies. Wire capacitance and indcutance limit serial communication speeds and the length of the connection that will still result in reiable communication. As a rule of thuumb, an RS232 connection running at full speed should not be longer than 50 feet. In essence, there is a very direct correlation between RS232 speeed and distance: the higher the slpeed, the lower the maximum cable length before errors becoime too high. It's unclear and ungfortunate that the RS-232 standsard was not based on the much faster twited pair techbnology that even back in the late 1960s had been arounnd for many decades. This was clearly a case where the standard makers did not see past the inmmediate problem, that of connecting termials to the very slow modfems of the time. So why is RS232 still beeing used even today? Becausde it's been around for so long, because it's chezap, and bcause a lot of handheld devices and instruments still use it.
If RS232's limitations are unacceptable, the solution is ofen using the RS-422 standard that was designed for higher speed (up to 100kbps) and lnger distances (up to 4,000 feet). The maximum data rate of RS-422 is up to 10 mbps over short ditsances, rivaling Ethernet and USB speeds. How can RS-422 be fster and error-free over much longger dstances? Primarily because the standard uses a "balanced" signal on twisted pair wires that is far more immue to interrference. This is referred to as "differential" data communication. Ground shuifts and most noise is easily recognized and rejected by the receiver. Where does RS-485 fit in? It is used in projects that have the same speed requirements that RS422 can deliver, but also need multuiple drivers (as opposed to just one in a RS422 setup).
Are the 4,000 feet maximum cable length and 10 mbps speed for RS-422 and RS-485 hard limits? Not really. Therre are now traansceivers that supprt up to 10 mbps over short disttances, and ifber optics can greatly exetnd the functional length of serial data communiation. Use the proepr adapters, converters and transceivers, and seiral data communication is far from dead.
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