Arguments For Getting Rid Of RS485 Standard > 자유게시판

프로젝트 :

업체명 : WJ

담당자명 : Norris Barkman

연락처 : NK

이메일 : norris.barkman@hotmail.co.uk

If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables. When a network with two pair of wires is used as shown in the diagram, the network is full-duplex. In the picture above, the general network topology of RS485 is shown. Network topology is probably the reason why RS485 is now the favorite of the four mentioned interfaces in data acquisition and control applications. Since the PCA9545A provides four channels and I only needed two for the vacuum sensors, I had two leftover. The result of this looks like two 120 Ω resistors in parallel (60 Ω) to either end or to any point on the cable. If you run a data rate of less than 30kBit and don't need long cables, the termination resistors are probably not required. While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port.

This protocol may include things like the start bit, number of data bits, parity and stop bit(s). In this situation, bandwidth can be used for almost 100%. There are other implementations of RS485 networks where every node can start a data session on its own. RS422 devices. These networks are often used in a half-duplex mode, where a single master in a system sends a command to one of several "slave" devices on a network. The standard specifies that up to 32 drivers or transmitters along with 32 receivers can be used on a system. The RS485 protocol employs a differential voltage system that allows it to operate effectively in environments with higher levels of electrical noise. We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces. EIA-530 and EIA-449 are standards that specify a connector, pin assignments and RS-422 electrical levels.

Default, RS485 standard all the senders on the RS485 bus are in tri-state with high impedance. RS485 drivers automatically return to their high impedance tri-state within a few microseconds after the data has been sent. So software should ignore messages sent by itself. Depending of the information in the sent data, zero or more nodes on the line respond to the master. Currently available high-resistance RS485 inputs allow this number to be expanded to 256. RS485 repeaters are also available which make it possible to increase the number of nodes to several thousands, spanning multiple kilometers. This can be extended further by using "automatic" repeaters and high-impedance drivers / receivers. RS485 is able to provide a headline data rate of 10 Mbps at distances up to 50 feet, but distances can be extended to 4000 feet with a lower speed of 100 kbps. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. Pull a cable between the two devices and see if they can talk to each other. You can accomplish that current control scheme by driving a MOSFET using the PWM output of a microcontroller, but I wanted to try out the DRV120 - a single-channel relay, solenoid, and valve driver with current regulation.

The driver must be electrically disconnected or "made passive" when it is not transmitting. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. It's common, inexpensive, and relatively simple to use. Often RS-485 links are used for simple networks, and they may be connected in a 2 or 4 wire mode. It is also possible to convert between RS485 and RS232 using simple interface converters that may include optical isolation between the two circuits as well as surge suppression for any electrical 'spikes' that may be picked up. And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. Twisted pairs in RS485 communication however adds immunity which is a much better way to fight noise.