Understanding RS-485 Communication > 자유게시판

The characteristic feature of the Daisy Chain topology is the sequential connection of each device along a single line, not forming a loop structure. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Ideally, the two ends of the cable will have a termination resistor connected across the two wires. If the total Unit Load is too high, the voltage drop across all devices will be too significant, leading to signal degradation. Unit Load measures how much load a device can impose on the communication link. The signal is a voltage change across a load as low as 1.5 volts differential from a driver, with a receiver detecting a differential voltage as minute as 200 millivolts. RS-485 communication involves a driver that produces signals and a receiver that detects them.

Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. When the connection is long, please use terminating resistors at both ends of the network, i.e., at the master and the farthest device. With your RS485 network, you can remotely monitor and control that pump from a control center. Devices utilizing RS485 can communicate with central control systems using communication protocols such as Modbus and ASCII. This makes utilizing RS485 simple. Much like RS232, RS485 doesn’t have an actual standard connector. Unlike Ethernet standards like TCP that repeatedly transmit packets until acknowledged (error correction), RS-485 signals transmit without requiring confirmation. RS-485 signals manifest as a differential voltage across the communication link. 0.2 is the minimum recommended voltage difference that receivers use to detect a change in polarity.

Since EIA-485 uses a voltage differential it can be used across large distances. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. EIA-485 does not specify or recommend any data protocol is only a specification for the transmission of the ones and zeros. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. RS-485 signals are used in a wide range of computer and automation systems. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. The entire daisy chain network is ultimately connected to a PLC (Programmable Logic Controller), facilitating data exchange between multiple devices and the central control system. This is possible since the HMI is connected to your PLC via RS485. The PLC is programmed to monitor the level of the water in the vessel.

What made it such an advancement over RS232 was the fact that it could transmit not only a single device to device transmission, but also a communications bus to connect multiple devices at once. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. When devices see a message with their address, they know to process the message and potentially send a response. Let’s talk about what we now know about RS485. If you are familiar with RS232, then you may already know a little bit about it. These are used in programmable logic controllers and on factory floors. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.

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