The standard USB RS485 converter / USB to RS422 adapter is a non-isolated adapter which is used for many business, office and personal devices. It is suitable for most applications, including industrial, commercial and any general office computer device. The RS-485 hardware generally receives on the receiver every byte that was transmitted by every device on the shared medium, including the local transmitter. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. See the instructions on the converter you are using. A DC/DC boost converter and capacitive inverter are integrated into the LTC2870 and LTC2871 to produce both positive and negative voltages used to support these drive levels while operating on a single 3V-5.5V supply. A separate logic supply pin VL allows the LTC2870 and LTC2871 to interface with any logic signals from 1.7V to 5.5V. All logic I/Os use VL as their high supply.

Port to modem communications usually use 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 baud. Since data rates on a PC serial port can vary from 300 baud (26mS for 8-bits) up to over 100k baud (0.08mS for 8-bits), the timer will have to either be limited in the baud rates that it can work with, or keep control of the RS-485 network for much longer than it needs. This was done since this protocol is a very common bit format used with serial communications. This appendix will attempt to explain what RS-232, RS-422, and RS-485 are and are not; then discuss one of the more common implementations of RS-422 and RS-485, asynchronous start-stop ASCII communication with a UART. The RS485 converter interface uses two wires for communication, one for transmitting and one for receiving data, which is referred to as a half-duplex communication converter. These old and slow microprocessors did not always have the horsepower to monitor the timing of both the incoming and outgoing bits, limiting the system to half-duplex. Normal cable variation, stub reflections and discontinuities have a far greater impact on signal integrity.

This condition is not actually a fault; it is a normal mode of operation in RS485. Figure 14 shows loopback operation. Optionally, the bus may be driven during loopback by simply enabling the appropriate driver. The LTC2870 and LTC2871 support this function without requiring a bus-biasing network, whether the bus is terminated or not. Figure 9 shows the LTC2870 or LTC2871 configured in a typical application with all of the required external components. Figure 9. Typical supply connections with external components shown. The only required external components are one 10µH inductor for the boost voltage and one 220nF cap for the voltage inversion, as well as the bypass caps on the generated VDD and VEE rails. These cables generally contain one or more twisted pairs as well as ground shields or a ground wire (sometimes called a drain wire). RS232 signals are driven on a single wire with respect to ground at levels that must exceed 5V and -5V. This kind of function could allow a user who wants to use a single rotary pot to control very large lighting loads that could not normally be controlled in such a format.

Figure 10 shows two LTC2870s, two LTC2871s or one of each sharing a single internal RS232 supply. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate). A final note on transmission lines is that the termination resistor should match the characteristic impedance of the cable. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. Some receivers cannot support this by themselves but require a resistor network to bias the differential signals on the bus in such a way that the receiver senses it as a high input. This is similar to the idle bus state but is truly a fault condition. Receivers that rely on bus biasing resistors to handle an idle bus condition do not respond properly to this type of fault. Idle Bus. All drivers on the bus are disabled with high impedance outputs. Shorted Bus. In this situation, the receiver inputs are shorted together.

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