B&B Electronics Asynchronous to Synchronous Converter 2010 Manuale Utente Scaricare il pdf (Pagina 13)

Department of Computer Science and Electronics

2.4. RS485

The main information sources about the RS-485 network are mentioned in the

reference. [8] [15]

EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical

specification of a two-wire, half-duplex, multipoint serial connection. The standard

specifies a differential form of signaling. The difference between the wires’ voltages is

what conveys the data. One polarity of voltage indicates logic 1 level; the reverse

polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid

operation, but any applied voltages between +12 V and -7 volts will allow correct

operation of the receiver.

EIA-485 only specifies electrical characteristics of the driver and the receiver. It

does not specify or recommend any data protocol. EIA-485 enables the configuration of

inexpensive local networks and multidrop communications links. It offers high data

transmission speeds (35 Mbit/s up to 10 m and 100 kbit/s at 1200 m). Since it uses a

differential balanced line over twisted pair (like EIA-422), it can span relatively large

distances (up to 4000 feet or just over 1200 meters).

EIA-485 drivers need to be put in transmit mode explicitly by asserting a signal

to the driver. This allows EIA-485 to implement linear topologies using only two lines

and between the ribbon cable. The purpose of this signal is to connect the driver to its

output terminals, A and B (Figure 2-8). If the "Enable" signal is OFF, one can consider

the driver as disconnected from the transmission line. An RS-485 driver must have the

"Enable" control signal when for instance in RS-422 driver is not always required.

The recommended arrangement of the wires is as a connected series of point-to-

point (multidropped) nodes, a line or bus, not a star, ring, or multiply-connected

network. Ideally, the two ends of the cable will have a termination resistor connected

across the two wires. Without termination resistors, reflections of fast driver edges can

cause multiple data edges that can cause data corruption. Termination resistors also

Figure 2-8. Balanced Differential Output Line Driver

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B&B Electronics Asynchronous to Synchronous Converter 2010 Manuale Utente Pagina 13