What Are the Speed and Distance Limits of Serial Communication?

By: Chris Robertson

One question that often comes up during discussions of the RS232, RS422 and RS485 serial data communication protocols are the speed and distance limits of the various protocols. This directly impacts the choice of protocol and therefore warrants discussion.

To reiterate, from an architectural standpoint, RS-232 is a uni-directional or bi-directional point-to-point link where a single channel is used for "half-duplex" communication or two independent channels are used for two way, or "full-duplex," communication. RS232 can also carry flow control and modem control signals. There are two types of connectors commonly used for serial communication. One has nine pins and one has 25 pins. RS232 signals consist of voltage relative to a common signal ground. A logic zero, also called "space," can be anywhere from 5 to 25 Volts. A logic one, also called "mark," can be anywhere between negative 5 and 25 Volts. This kind of communication is referred to as "unbalanced."

The RS-232 standard defines a maximum speed of just 20 kbps (kilobits per second). 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 interference occurs between adjacent wires. Wire capacitance and inductance limit serial communication speeds and the length of the connection that will still result in reliable communication. As a rule of thumb, an RS232 connection running at full speed should not be longer than 50 feet. In essence, there is a very direct correlation between RS232 speed and distance: the higher the speed, the lower the maximum cable length before errors become too high. It's unclear and unfortunate that the RS-232 standard was not based on the much faster twisted pair technology that even back in the late 1960s had been around for many decades. This was clearly a case where the standard makers did not see past the immediate problem, that of connecting terminals to the very slow modems of the time. So why is RS232 still being used even today? Because it's been around for so long, because it's cheap, and because a lot of handheld devices and instruments still use it.

If RS232's limitations are unacceptable, the solution is often using the RS-422 standard that was designed for higher speed (up to 100kbps) and longer distances (up to 4,000 feet). The maximum data rate of RS-422 is up to 10 mbps over short distances, rivaling Ethernet and USB speeds. How can RS-422 be faster and error-free over much longer distances? Primarily because the standard uses a "balanced" signal on twisted pair wires that is far more immune to interference. This is referred to as "differential" data communication. Ground shifts 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 multiple 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. There are now transceivers that support up to 10 mbps over short distances, and fiber optics can greatly extend the functional length of serial data communication. Use the proper adapters, converters and transceivers, and serial data communication is far from dead.

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Chris Robertson is an author of Majon International, one of the worlds MOST popular internet marketing companies. For tips/information, click here: RS422 Converters
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