B&B公司的资料是12kΩ，而TI公司的资料是15kΩ。
B&B公司(没有链接，请原谅)：
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
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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
