作者： iC921 于 2006/5/15 8:39:00 發(fā)布：
不同的情況有不同的要求
下面就有正規(guī)、精確的、大致的兩種情形：
| 研討翻譯 RS-422 and RS-485 Application Note 第2章 -2- |
Biasing an RS-485 Network
RS485網(wǎng)絡(luò)的偏置
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When an RS-485 network is in an idle state, all nodes are in listen (receive) mode. Under this condition there are no active drivers on the network, all drivers are tristated. Without anything driving the network, the state of the LINE is UNKNOWN. If the voltage level at the RECEIVER’s A and B inputs is less than ±200 mV the LOGIC level at the OUTPUT of the RECEIVERs will be the VALUE of the last bit received. In order to maintain the proper idle voltage state, bias RESISTORs must be applied to force the data LINEs to the idle condition. Bias RESISTORs are nothing more than a pullup RESISTOR on the data B LINE (typically to 5 volts) and a pulldown (to ground) on the data A LINE. Figure 2.4 illustrates the placement of bias RESISTORs on a transceiver in a two-wire configuration. Note that in an RS-485 four-wire configuration, the bias RESISTORs should be placed on the RECEIVER LINEs. The VALUE of the bias RESISTORs is dependent on termination and NUMBER of nodes in the SYSTEM. The goal is to generate enough DC bias current in the network to maintain a MINIMUM of 200 mV between the B and A data LINE. Consider the following two examples of bias RESISTOR calculation.
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RS-485網(wǎng)絡(luò)處于空閑狀態(tài)時，所有的節(jié)點都成為聽模式(接收模式)。在這種情況下，所有發(fā)送器都是第三態(tài)，網(wǎng)絡(luò)上沒有任何工作的驅(qū)動器。沒有任何(驅(qū)動器)驅(qū)動網(wǎng)絡(luò)，則線路狀態(tài)不明(未知)。如果接收器輸入端A和B間的電平低于±200mV，接收器輸出的邏輯電平將被當作末位值接收起來。為保持適當?shù)目臻e電壓狀態(tài)，就必須使用偏置電阻強迫數(shù)據(jù)線維持線路(處于正確的)空閑條件。偏置電阻不過是數(shù)據(jù)線B(典型為+5V)的上拉電阻和數(shù)據(jù)線A(接到“地”)的下拉電阻，如圖2.4所示的雙線系統(tǒng)，詳細示出了收發(fā)器的偏置電阻。但要注意，對于4線系統(tǒng)，偏置電阻應(yīng)當放在接收器一端。偏置電阻的值，取決于是不是在終端(termination)和由系統(tǒng)節(jié)點數(shù)(的多少)來確定。使用終端電阻的目的，是讓網(wǎng)絡(luò)發(fā)生足夠的直流偏置電流，以維持數(shù)據(jù)線A和B間的最小電壓在200mV(以上)。參見以下兩個有關(guān)偏置電阻計算的例子。
［譯者語］The VALUE of the bias RESISTORs is dependent on termination and NUMBER of nodes in the SYSTEM一句似乎很有E文特色，就著它“譯”出來。只是其中關(guān)鍵是“termination”一詞如何能與“VALUE”有關(guān)？ |
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Figure 2.4 - Transceiver with Bias Resistors Example 1. 10 node, RS-485 network with two 120 W termination RESISTORs
例1 含兩個120Ω終端電阻的10節(jié)點RS-485網(wǎng)絡(luò)
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Each RS-485 node has a load impedance of 12KΩ. 10 nodes in parallel give a load of 1200 Ω. Additionally, the two 120 Ω termination RESISTORs result in another 60 W load, for a total load of 57 Ω. Clearly the termination RESISTORs are responsible for a majority of the loading. In order to maintain at least 200mV between the B and A LINE, we need a bias current of 3.5 mA to flow through the load. To create this bias from a 5V supply a total series resistance of 1428 Ω or less is required. Subtract the 57 Ω that is already PART of the load, and we are left with 1371 Ω. Placing half of this VALUE as a pullup to 5V and half as a pulldown to ground gives a maximum bias RESISTOR VALUE of 685Ω for each of the two biasing RESISTORs.
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各RS-485節(jié)點有負載阻抗為12kΩ，10個節(jié)點并聯(lián)后得出的阻抗為1200Ω，另外，兩個120Ω的線路終端電阻形成的另一個負載為60Ω，因此，總負載電阻為57Ω。明顯地，終端電阻將是線路的主要負載。為保持A、B線間至少有200mV的電壓，這就需要3.5mA的偏置電流流過(這57Ω的)負載。用5V電源進行偏置時，所要串聯(lián)的電阻最大為1428Ω�？鄢呀�(jīng)有的57Ω負載電阻，剩下的電阻為1371Ω。如將其值的一半接到5V電源做上拉電阻，將另外的一半拉接到地做下拉電阻，則各個節(jié)點的兩邊偏置電阻最大總共為685Ω。
［譯者語］最后一句說的是上拉或下拉電阻的并聯(lián)值而不是直接利用這個值。這一點比較容易誤會，而目前看到的資料又沒有對此加以說明的，因而特別強調(diào)之。 |
Example 2. 32 node, RS-485 network without termination
例2：沒有終端電阻的32節(jié)點網(wǎng)絡(luò)
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Each RS-485 node has a load impedance of 12KΩ. 32 nodes in parallel gives a total load of 375 Ω. In order to maintain at least 200 mV across 375Ω we need a current of 0.53 mA. To generate this current from a 5V supply requires a total resistance of 9375W maximum. Since 375 Ω of this total is in the RECEIVER load, our bias RESISTORs must add to 9KΩ or less. Notice that very little bias current is required in SYSTEMs without termination.
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各RS-485節(jié)點的負載阻抗為12kΩ，32節(jié)點并聯(lián)所得的總負載為375Ω。為保持375Ω負載的電壓至少為200mV，需要的電流為0.53mA。通過5V電源發(fā)生這樣的電流，所需的負載電阻最大為9375Ω。因有375Ω是接收口器總負載(固有)，因此，總的偏置電阻必須增加到9kΩ或更小。注意，系統(tǒng)沒有終端電阻時所需的偏置電流非常小。
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［025］ |
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Bias RESISTORs can be placed anywhere in the network or can be split among multiple nodes. The parallel combination of all bias RESISTORs in a SYSTEM must be equal to or less than the calculated biasing requirements. B&B ELECTRONICS uses 4.7KΩ bias RESISTORs in all RS-485 products. This VALUE is adequate for most SYSTEMs without termination. The SYSTEM designer should always calculate the biasing requirements of the network. Symptoms of under biasing range from decreased noise immunity to complete data failure. Over biasing has less effect on a SYSTEM, the primary result is increased load on the drivers. Systems using PORT powered RS-232 to RS-485 converters can be sensitive to over biasing.
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偏置電阻可以放在網(wǎng)絡(luò)的任何位置或分別放到網(wǎng)絡(luò)中的多處節(jié)點。系統(tǒng)中，所有偏置電阻并聯(lián)后的總阻抗必須等于或小于計算出的偏置要求值。B&B Electonics公司所有的RS-485產(chǎn)品用4.7KΩ偏置電阻。此值適用于多數(shù)沒有終端電阻的系統(tǒng)。系統(tǒng)設(shè)計師務(wù)必計算網(wǎng)絡(luò)的偏置要求(值)。在偏置范圍以下，則存在降低噪聲的抗干擾性導致數(shù)據(jù)失效的symptoms。
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Extending the Specification
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Some SYSTEMs require longer distances or higher NUMBERs of nodes than supPORTed by RS-422 or RS-485. Repeaters are commonly used to overcome these barriers. An RS-485 repeater such as B&B ELECTRONICS’ 485OP can be placed in a SYSTEM to divide the load into multiple segments. Each “refreshed” signal is capable of driving another 4000 feet of CABLE and an additional 31 RS-485 loads.
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性能擴展
往往有一些系統(tǒng)需要比RS-422或RS-485所支持的有更長的距離或更多的節(jié)點數(shù)。要突破這類局限，通常要用到重復器。象B&B公司的485OP型這樣的RS-485總線重復器，可以插入到系統(tǒng)中將負載分割成許多段，各“復原”后的信號有驅(qū)動另一段4000ft電纜和帶動另外31個RS-485負載的能力。
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Another method of increasing the NUMBER of RS-485 nodes is to use low load type RS-485 RECEIVERs. These RECEIVERs use a higher input impedance to reduce the load on the RS-485 drivers to increase the total NUMBER of nodes. There are currently half and quarter load integrated CIRCUIT RECEIVERs available, extending the total allowable NUMBER of nodes to 64 and 128.
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| 4樓： |
>>參與討論 |
作者： iC921 于 2006/5/15 8:40:00 發(fā)布：
注意它也包括了上拉下拉兩種情形
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| 5樓： |
>>參與討論 |
作者： zlucas 于 2006/5/16 7:57:00 發(fā)布：
謝謝
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