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1 (MARK or OFF) state. b. When the A terminal of the driver is positive with respect to the B terminal, the line is in a binary

0 (SPACE or ON) state. Figure 1.5 shows the condition of the voltage of the balanced line for an RS-232 to RS-422 converter when the line is in the idle condition or OFF state. It also shows the relationship of the A and B terminals of an RS-422 system and the - and + terminal markings used on many types of equipment. The A terminal is equivalent to the - designation, and the B terminal equivalent to the + designation. The same relationship shown in Figure 1.5 also applies for RS-485 systems. RS-422 can withstand a common mode voltage (Vcm) of ±7 volts.??Common mode voltage is defined as the mean voltage of the A and B terminals with respect to signal ground. Figure 1.4: Typical RS-422 Four Wire Network Figure 1.5: Relationship between EIA Standard A and B terminals on an RS-422 or RS-485 Device and + and - Identification Convention NOTE: Under idle conditions it is possible to determine which terminal is A and which is B . 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. Figure 1.6: typical RS-485 two wire multidrop network An RS-485 network can also be connected in a four-wire mode as shown in Figure 1.7. Note that four data wires and an additional signal ground wire are used in a four-wire connection. In a four-wire network it is necessary that one node be a master node and all others be slaves. The network is connected so that the master node communicates to all slave nodes. All slave nodes communicate only with the master node. This network has some advantages with equipment with mixed protocol communications. Since the slave nodes never listen to another slave response to the master, a slave node cannot reply incorrectly to another slave node. Figure 1.7: typical RS-485 four wire multidrop network Tristate Control of an RS-485 Device using RTS As discussed previously, an RS-485 system must have a driver that can be disconnected from the transmission line when a particular node is not transmitting. In an RS-232 to RS-485 converter or an RS-485 serial card, this may be implemented using the RTS control signal from an asynchronous serial port to enable the RS-485 driver. The RTS line is connected to the RS-485 driver enable such that setting the RTS line to a high (logic 1) state enables the RS-485 driver. Setting the RTS line low (logic 0) puts the driver into the tristate condition.??This in effect disconnects the driver from the bus, allowing other nodes to transmit over the same wire pair. Figure 1.8 shows a timing diagram for a typical RS-232 to RS-485 converter. The waveforms show what happens if the VRTS waveform is narrower than the data VSD. This is not the normal situation, but is shown here to illustrate the loss of a portion of the data waveform. When RTS control is used, it is important to be certain that RTS is set high before data is sent. Also, the RTS line must then be set low after the last data bit is sent. This timing is done by the software used to control the serial port and not by the converter. Figure 1.8: Timing Diagram for RS-232 to RS-485 Converter with RTS Control of RS-485 Driver and Receiver Note: