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RS-485的單位負(fù)載到底是多少？ |
| 作者：iC921 欄目：通信技術(shù) |
我正在看的一份B&B公司的資料是12kΩ，而TI公司的資料是15kΩ。 ------- B&B公司(沒(méi)有鏈接，請(qǐng)?jiān)?： Example 1. 10 node, RS-485 network with two 120 Ω termination RESISTORs 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 Ω 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. RS-422/485 Application Note B&B ELECTRONICS -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 ============= TI公司： The Unit Load TIA/EIA-485-A defines a Unit Load as a 15-kΩ; RESISTOR connected to a -3-V or 5-V source (see Figure 2). The -3-V case applies for positive input current and the 5-V case for negative bus current. The definition and MODEL are valid for input voltages from -7 V to 12 V to account for driver outputs between 0 V and 5 V with up to ±7 V of common-mode noise voltage between a driver and RECEIVER. http://focus.ti.com.cn/cn/lit/an/slla166/slla166.pdf * - 本貼最后修改時(shí)間：2006-1-18 18:00:29 修改者：iC921 |
| 2樓： | >>參與討論 |
| 作者： iC921 于 2006/1/18 19:40:00 發(fā)布：
估計(jì)是12kΩ 但不能確信 422 and 485 Standards Overview and SYSTEM Configurations ======================================================== U.L. Concept (VIA and VIB Applied; IIA and IIB Measured) As with the 422, a maximum limit on the I/V characteristic must be placed on the RECEIVER, driver (off state), and transceiver to ensure a maximum load on the bus when all 32 U.L.s are utilized. With the voltage VIA (VIB) ranging from –7 V to 12 V, while VIB (VIA) is grounded, the resulting input current IIA (IIB) should remain within the shaded region in both power-on and power-off conditions (see Figure 10). A DEVICE with input characteristics that fall within the shaded region conforms to having a 1-U.L. characteristic. The 485 STANDARD specifies the capability to sustain up to 32 U.L.s. The 485 often is thought of as a 12-kΩ load STANDARD. Because the OUTPUT current of the driver is dependent on loading, a design that requires a large NUMBER of stations (drivers, RECEIVERs, or transceivers) attached to the bus needs a larger load resistance to allow more connections. For example, a 0.5-U.L. transceiver can be placed up to 64 times on a bus, because this configuration complies with the maximum 32-U.L. specification. http://focus.ti.com.cn/cn/lit/an/slla070c/slla070c.pdf |
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| 3樓： | >>參與討論 |
| 作者： iC921 于 2006/1/19 12:13:00 發(fā)布：
TIA/EIA-485-A標(biāo)準(zhǔn)找不到 |
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