이름 : Leopoldo Brinkman
이메일 : leopoldo_brinkman@facebook.com
연락처 :
예식일 : How Fast can you Go?
문의내용:

A conversation with a cable manufacturer can help to define the best available cable for your application. In addition to using the best cable, there are many different drivers, receivers, and transceivers available. RS-422's requirement is very similar except the resistors are specified as 499Ω rather than a ratio of UL. RS-422's annex has a chart of empirical data using 24 AWG copper UTP telephone cable. A receipt response verifying valid data received can be sent back to the master. The annex says this connection can be wired directly or made through a 100 Ω resistor. The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. If you are trying to get two ancient pieces of equipment (both of which use true RS-232 drivers and receivers) to work with a long cable, the capacitance limit in RS-232 may apply, but RS-232 does not take bit rate into account. This should give you an approximate limit for that cable in feet. 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.

If the length is short enough and the bit rate is low enough, completely unbalanced loose wires will work. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable. 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. Section 1.3, Data Signalling Rates, RS485 standard states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. Data rate is usually the primary factor. The primary difference between RS-422 and RS-485 is that RS-485 requires two wires in a single pair which all devices transmitters and receivers are connected to, and RS-422 typically uses two or more pair in which only one transmitter and up to ten receivers are connected to a single pair. Shielded cable increases the capacitance between wires and reduces the total cable length.

Low data rates are primarily limited by the DC resistance of the cable (the effects of the DC resistance of the cable are made worse if a termination resistor is used) and high data rates are limited by the AC effects of the cable on signal quality. 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. Devices may be designed to operate at lower data rates for "economically specific applications". 200mV. This can cause a problem if the RS-485 network is using a UART to transmit data. If you have to know that it will work before you pull the cable, then get the required length of the cable and move the two devices next to each other, connect them and see if they can talk. Stub length, termination, and biasing resistors can have a significant impact on the performance of the network. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered.

RS-485 does not define a way for a device to detect that the data it transmitted has been corrupted. Anyone who has seen a device go up in smoke, is likely to be very vehement on how to, and how not to wire grounds. For long cables, heavier gauge wire may be needed to reduce resistance. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. At the other end of the cable is (typically) the same thing -- the connectors, the RS-485 interface IC, and a UART inside a microcontroller. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω. The annex of RS-485 states that "When employed, the shield shall be connected only to frame ground at either or both ends depending on the specific application." Shields usually have a lot more copper (and/or aluminum) than a single wire and can therefore carry a lot more current. If reducing the bit rate is not an option, then you could try an RS-232 to RS-485 converter at both ends. Try to find the capacitance of the cable being used (in pF/foot) and divide 2500 by it.