What To Expect From RS485 Standard? > 자유게시판

This means that in a master/slave configuration, a master can talk to multiple slaves, but only one slave can talk back to the master. A receipt response verifying valid data received can be sent back to the master. A CRC or checksum can be added to the end of the data being transmitted. Its annex also states "It is desirable for proper operation over the interchange cable that the DC wire resistance not exceed 25 Ω per conductor." Any cable that meets the capacitance requirement can be used. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. I.e. only one device can talk to the other devices on the RS-485 network at a time. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices.

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. When a network with two pair of wires is used as shown in the diagram, the network is full-duplex. Each pair has two wires to which a driver and one more receivers are connected. This standard specifies the electrical characteristics of a single transmitter and up to ten receivers on a single pair of wires. The RS-485 standard includes the voltage levels of the two wires when a binary 0 or 1 is on the two RS-485 wires, but specifically excludes the logic function of the generator or receiver. EIA-530 and EIA-449 are standards that specify a connector, pin assignments and RS-422 electrical levels. RS-422 includes the voltage levels of the two wires when a binary 0 or 1 is on the RS-422 lines, but specifically excludes the logic function of the generator or receiver. The signals appear differentially on two wires of each pair. This standard is for a network of a single balanced pair of wires. 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.

Devices may be designed to operate at lower data rates for "economically specific applications". Clearly not all of these circuits or signals are included in IBM’s implementation (very few implementations use secondary data lines or many of the other circuits). The RS-485 standard title is "Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems". A decent oscilloscope that is isolated from earth ground (battery operated or powered by an unplugged UPS (if you're good enough with a scope you can also subtract channel 1 from channel 2 to see the differential waveform, or use a differential probe, etc.)) can be a very useful troubleshooting tool. RS-485, like RS-422, can be made full-duplex by using four wires. The inductance, resistance, and capacitance of the wires are modeled below. 3V are undefined by the standard. The IBM PC serial port is only partially described by the RS-232 standard. RS-232 specifies two connectors, a 25-pin (DB-25) and an alternate 26-pin connector (pin 26 is "No Connection").

This is a rather convoluted way of saying pin 7 of the DB-25 connector is used as a signal common. The RS-232 standard defines a DB-25 and a 26-pin alternate connector, the function of each of the pins in the connector (called circuits), and the electrical characteristics of the signals that are on the "circuits". RS-232 allows for both a transmit and receive circuit. This allows RS-485 to transmit over longer distances than RS-232. RS-232 uses point-to-point unidirectional signal wires: There are only two devices connected to a RS-232 cable. I.e. the two devices on an RS-422 network can talk to each other at the same time. There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. There are numerous mentions of "empirically determined" in these standards. Other standards such as EIA-530 reference RS-422 for electrical signal levels, and include multiple signals on multiple pairs of wires. Figure 1 of the RS-485 standard is a diagram (not shown for copyright concerns) which shows the "balanced interconnecting cable" as a transmission line of two wires. A third point on the driver shown as "C" is labeled as a common, but is not shown connecting to any wire.

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