What is RS485 and how is it used in Industrial Control Systems? > 자유게시판

The SMINI includes circuitry for two 24-pin output cards and one 24-pin input card. 5Vdc card traces, whereas the larger ground traces are used in the current sinking case. In reality either or both the driver or receiver are going to meet the more modern RS-574 requirements and none of the RS-232 limits will apply. In reality, if you are using relatively modern equipment (say 1990 or later) and a low baud rate, line lengths of 1,000 feet (300 m) or more are possible. This limits the load termination and is an argument against using 100 Ω cable. There are no limits given in time units, so there is no minimum or maximum bit rate associated with RS-485. For existing installations or setups with previously designed modules that are challenging to update, implementing this method may not be feasible. The reason for this is that different installations have different grounding requirements. The 5V supply is going to have capacitors on its output. Now look at the driver schematic symbol you should notice that the "A" output is the non-inverting output and the "B" output is inverting.

This is not technically correct since RS-232 requires the use of voltages outside those of normal logic, but the input and output of an RS-232 driver look inverted on an oscilloscope, and most data sheets show the driver as an inverter. RS-232 is very clear that is intended for a maximum bit rate of 20kbit/s. The forward recommends EIA/TIA-530, EIA/TIA-561, and EIA/TIA-574 for operation at higher bit rates. The forward to RS-485 references TSB-89 which has topics including data signaling rate vs. Then it references section 2.1.4 where the maximum capacitance of the receiver side of the interface point, including the cable, shall not exceed 2500pF. Then suggests seeing appendix A for guidance (again, the appendix states that it is not a formal part of the standard). The appendix discusses capacitance and resistance of the cable, then gives an example calculation where the capacitance of the cable per foot (30pF/foot) multiplied by the cable's length, plus the capacitance of the receiver (100pF) gives a maximum cable length of 80 feet. Try to find the capacitance of the cable being used (in pF/foot) and divide 2500 by it. Empirically determined means try it and see if it works. This means that low data rates can handle an improperly terminated (or even unterminated) cable better than high data rates.

In addition, RS485 supports multi-drop communication, which means that multiple devices can be connected to the same communication line. RS485 can handle up to 32 connected devices. RS485 uses balanced transmission and differential reception, so it has the ability to suppress common mode interference. If you’re looking for a low data transmission speed over a short distance, the RS232 will suffice, even though it is slightly archaic. RS-485 has even less information. The RS-485 standard requires a minimum performance, but many drivers exceed this performance and some have quirks such as slew rate limiting. Slew rate limited drivers will improve signal quality significantly with an unterminated network. 3V as a logic "0", a signal lower than -3V as a logic "1", a TTL level of 5V as a logic positive, and 0 as a logic negative. Devices may be designed to operate at lower data rates for "economically specific applications". The annex discusses the fact that many applications can handle greater amplitude and timing distortion, and that practical experience has shown that the cable length can be extended to several kilometers at lower data rates.

A conversation with a cable manufacturer can help to define the best available cable for your application. Add the cost of pulling the cable and long networks can get very expensive. And, of course, the installation can make or break the network performance. If they don't communicate, reducing the bit rate may make the two devices function together. Pull a cable between the two devices and see if they can talk to each other. A kilometer of POTS cable can cost a lot, two kilometers twice as much. A gateway that converts the RS-485 data to run over an existing network may cost less in the end than running a kilometer of cable. Then the software will work unchanged when connected to a full-duplex RS-485 network, a RS-232 network, and a variety of other communication media. If you keep the cable length at 70% or 80% of this limit you should expect the network to work with a true RS-232 driver and receiver. All of the application guides and data sheets that say RS-485 has a limit of 1200 meters or 10Mbit are flat out wrong. The legends and folklore, not to mention the flat out wrong information that has grown around the line length and data rate limits inherent in RS-485 are truly astounding.

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