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Ошибка преобразователя частоты

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MessagePosté le: Sam 7 Avr - 15:24 (2018)    Sujet du message: Ошибка преобразователя частоты Répondre en citant

voltage at about -0.7V, and the isolated RS-232 output is negative. In the other direction, the circuits are simi- 75 Chapter 5 lar to the TTL-to-RS-232 circuit except that no 74LS14 inverter is needed because the non-isolated RS-232 interface inverts the signal. For VCC1 in the RS-232 to RS-232 circuit, you can use a positive output at DTR or RTS if the signal is otherwise unused. The cable on the VCC1 side of this circuit should be short. Typical turn-on and turn-off times for phototransistors is several microseconds, which should cause no problems at data rates of 20 kbps or less. For fast bit rates, look for a photodiode with switching times of 1/10 or less of the bit width. Another way to achieve an isolated interface is to use separate, isolated supplies for the RS-232 voltages. Maxim Integrated Products’ MAX250 and MAX251 enable creating an isolated interface with the addition of four optocouplers and a transformer. Fiber Optics A different way to isolate a link is to use fiber-optic cable in place of copper wire. Fiber-optic cable encodes data as the presence or absence of light or via more complex encoding methods. Fiber-optic cable has several advantages. The signals are immune to ground noise and electromagnetic interference and generate no electromagnetic interface. A cable typically can run 1 to 2.5 miles before requiring a repeater. The main disadvantage is expense, including the need for special connectors and tools. Debugging Tools A breakout box, voltmeter, and oscilloscope can be helpful when setting up and debugging circuits for serial communications. Using a Breakout Box A breakout box Figure 5-8 connects in series with an RS-232 cable and displays the status of each line in the cable. LEDs indicate the logic states of the signals. Some boxes also have switches and jumper-wire connections that enable you to rewire the interface in any configuration. For example, you can experi- 76 Designing RS-232 Links Figure 5-8: Breakout boxes often include

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MessagePosté le: Sam 7 Avr - 15:24 (2018)    Sujet du message: Publicité

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