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6 RS-422/485 Application Note

 Copyright B&B Electronics -- Revised October 1997

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

EIA Standard RS-485 Data Transmission

The RS-485 Standard permits a balanced transmission line to be shared in

a party line or multidrop mode. As many as 32 driver/receiver pairs can share

a multidrop network. Many characteristics of the drivers and receivers are the

same as RS-422. The range of the common mode voltage Vcm that the driver

and receiver can tolerate is expanded to +12 to -7 volts. Since the driver can

be disconnected or tristated from the line, it must withstand this common mode

voltage range while in the tristate condition. Some RS-422 drivers, even with

tristate capability, will not withstand the full Vcm voltage range of +12 to -7

volts.

Figure 1.6 shows a typical two-wire multidrop network. Note that the

transmission line is terminated on both ends of the line but not at drop points

in the middle of the line. Termination should only be used with high data rates

and long wiring runs. A detailed discussion of termination can be found in

Chapter 2 of this application note. The signal ground line is also

recommended in an RS-485 system to keep the common mode voltage that the

receiver must accept within the -7 to +12 volt range. Further discussion of

grounding can be found in Chapter 3 of this application note.

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B&B Electronics RS-422 Especificaciones Pagina 9