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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. 6 RS-422/485 Application Note
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