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參數資料
| 型號： | RS-485 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | RS-422 AND RS-485 APPLICATION NOTE |
| 中文描述： | RS - 422和RS - 485接口應用筆記 |
| 文件頁數： | 12/44頁 |
| 文件大小： | 628K |
| 代理商： | RS-485 |
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RS-422/485 Application Note
9
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
An RS-485 network can also be connected in a four-wire mode as shown
in Figure 1.7. Note that four data wires and an additional signal ground wire
are used in a “four-wire” connection. In a four-wire network it is necessary
that one node be a master node and all others be slaves. The network is
connected so that the master node communicates to all slave nodes. All slave
nodes communicate only with the master node. This network has some
advantages with equipment with mixed protocol communications. Since the
slave nodes never listen to another slave response to the master, a slave node
cannot reply incorrectly to another slave node.
Tristate Control of an RS-485 Device using RTS
As discussed previously, an RS-485 system must have a driver that can
be disconnected from the transmission line when a particular node is not
transmitting. In an RS-232 to RS-485 converter or an RS-485 serial card,
this may be implemented using the RTS control signal from an asynchronous
serial port to enable the RS-485 driver. The RTS line is connected to the RS-
485 driver enable such that setting the RTS line to a high (logic 1) state
enables the RS-485 driver. Setting the RTS line low (logic 0) puts the driver
into the tristate condition. This in effect disconnects the driver from the bus,
allowing other nodes to transmit over the same wire pair. Figure 1.8 shows a
timing diagram for a typical RS-232 to RS-485 converter. The waveforms
show what happens if the VRTS waveform is narrower than the data VSD.
This is not the normal situation, but is shown here to illustrate the loss of a
portion of the data waveform. When RTS control is used, it is important to
be certain that RTS is set high before data is sent. Also, the RTS line must
then be set low after the last data bit is sent. This timing is done by the
software used to control the serial port and not by the converter.
When an RS-485 network is connected in a two-wire multidrop party
line mode, the receiver at each node will be connected to the line (see Figure
1.6). The receiver can often be configured to receive an echo of its own data
transmission. This is desirable in some systems, and troublesome in others.
Be sure to check the data sheet for your converter to determine how the
receiver “enable” function is connected.
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