May 2005
Beginner to intermediate
398 pages
12h 12m
English
Content preview from Designing Embedded Hardware, 2nd Edition
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RS-422
Unlike RS-232C, which is referenced to local ground, RS-422 uses the difference between two lines, known as a twisted pair or a differential pair , to represent the logic level. Thus, RS-422 is a balanced transmission, or, in other words, it is not referenced to local ground. Any noise or interference will affect both wires of the twisted pair, but the difference between them will be less affected. This is known as common-mode rejection . RS-422 can therefore carry data over longer distances and at higher rates with greater noise immunity than RS-232C. RS-422 can support data transmission over cable lengths of up to 1,200 meters (approximately 4,000 feet).
Figure 9-9 shows a basic RS-422 link, where a driver (D) of one embedded system is connected to a receiver (R) of another embedded system via a twisted pair. The resistor, Rt, at receiving end of the twisted pair is a termination resistor. It acts to remove signal reflections that may occur during transmission over long distances, and it is required. Rt is nominally 100-120 Ω.
Figure 9-9. RS-422
The voltage difference between an RS-422 twisted pair is between ±4 V and ±12 V between the transmission lines (Figure 9-10). RS-422 is, to a degree, compatible with RS-232C. By connecting the negative side of the twisted pair to ground, RS-422 effectively becomes an unbalanced transmission. It may then be mated with RS-232C. Since ...
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