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ARTICLES


Serial Communication Speed and Distance Limits

When discussing RS232, RS422 and RS485 serial data communication protocols, questions that come up are related to 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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SIMA S. ENTERPRISES LLC
www.rs232-converters.com
email:sales@simaenterprises.com
PHONE:1-877-647-7434

FAX: 1-732-749-3598

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Quality Guarantee - SerialComm converters are low priced, high quality products and made to last a lifetime. They are manufactured in a ISO-9001 certified facility using surface mount technology with lead free components.  All converters are pretested prior to shipment and are fully guaranteed for 5 years.  There are no converters, Commercial or Industrial that surpass SerialComm's quality standards at any price tag. Satisfaction Guarantee - We offer a 100% 30 Day Money Back Guarantee which most companies do not offer.  If for any reason, you 're not satisfied with your purchase within 30 days, you can return it with no penalties.  We will return the full value of the order minus shipping costs. We do not reimburse express shipping fees.  We do not add restocking fees when returning a purchase within 30 days. Manufacturer's WarrantySerialComm offers a 5-year Workmanship Warranty.  We are so confident our product are built to last, within a 5 year period from the time of purchase, if a rare event of a malfunction should occur; we will replace your product for free.  Fill out our RMA form and we will issue you a RMA number for return of your product.   We do not issue refunds after 30 days of purchase. Express Shipping Guarantee - We specialize in shipping express orders on-time.  If FEDEX express shipping is selected before 6:30 PM E.S.T. and the items are in stock; we guarantee that we will get the order delivered to FEDEX on-time for the scheduled express delivery.  If we fail do to so, we will ship you the package the next day and credit you the full value of the order. (LIMITED TO $100 PER ORDER).  Please refer to our Terms for full details.
 

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