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Ayrıntılar Basics of the RS-485 Standard
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İsimBasics of the RS-485 Standard
AçıklamaBasics of the RS-485 Standard This information touches on some of the most commonly asked aspects of RS-485 communications. B&B Electronics has a free application note available on RS-422/485 that gives a more complete picture of RS-485 networks. Request B&Bs RS-422/485 Application Note, available by mail or on our websites, www.bb-elec.com or www.bb-europe.com What is an RS-485 network? RS-485 allows multiple devices (up to 32) to communicate at half-duplex on a single pair of wires, plus a ground wire (more on that later), at distances up to 1200 meters (4000 feet). Both the length of the network and the number of nodes can easily be extended using a variety of repeater products on the market. How does the hardware work? Data is transmitted differentially on two wires twisted together, referred to as a "twisted pair." The properties of differential signals provide high noise immunity and long distance capabilities. A 485 network can be configured two ways, "two-wire" or "four-wire." In a "two-wire" network the transmitter and receiver of each device are connected to a twisted pair. "Four-wire" networks have one master port with the transmitter connected to each of the "slave" receivers on one twisted pair. The "slave" transmitters are all connected to the "master" receiver on a second twisted pair. In either configuration, devices are addressable, allowing each node to be communicated to independently. Only one device can drive the line at a time, so drivers must be put into a high-impedance mode (tri-state) when they are not in use. Some RS-485 hardware handles this automatically. In other cases, the 485 device software must use a control line to handle the driver. (If your 485 device is controlled through an RS-232 serial port, this is typically done with the RTS handshake line.) A consequence of tri-stating the drivers is a delay between the end of a transmission and when the driver is tri-stated. This turn-around delay is an important part of a two-wire...
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Ekleme tarihi: 15.10.2008 16:13
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En son güncelleme 14.10.2008 16:14
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