Questions For/About Rs485 Cable > 자유게시판

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EIA standards where previously marked with the prefix "RS" to indicate recommended standard; however, the standards are now generally indicated as "EIA" standards to identify the standards organization. Below are the specifications for RS232, RS423, RS422, and RS485. RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. They should generally not be needed, except if you use long cables, multiple RS485 devices, and resistive termination. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. EIA-485 only specifies electrical characteristics of the driver and the receiver. Receiver Input Resistance (Ohms), (1 Standard Load for RS485) 3k to 7k 4k min. Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow.

RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. There are surface-mount resistor pads on the Docking Panel to bring out the RS485 signals to the DB9 Serial 1 Connector. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. The local and remote must share a common ground, so all serial cables include at least one ground conductor. Typically one device (node) is addressed by the host computer and a response is received from that device. The BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. The hardware detects the start-bit of the transmission and automatically enables (on the fly) the RS485 transmitter.

An RS485 cable can carry data upto 4000ft (or 1.2km) @ 100kbps as shown below. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). The connection between two or more elements (drivers and receivers) should be considered a transmission line if the rise and/or fall time is less than half the time for the signal to travel from the transmitter to the receiver. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg.

With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. Rather, it relies on software handshaking via transmission of XON/XOFF characters (ascii 0x11 and 0x13, respectively) to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. So, for eight data bits with a parity bit, M would be set (equal to one) in order to add an extra bit to each byte transmitted, and PE would be set in order to make that extra bit be used as a parity bit. All of the serial ports are supported by pre-coded C-language software drivers that make it easy to exchange data. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit. A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream.