请问RS485通信，7M速率时，电缆线最长可多少?

<br /><br />&nbsp;&nbsp;&nbsp;&nbsp;如果用SP491，需要接多大的电阻。&nbsp;<br />&nbsp;<br />

详细请进入：<a href="http://blog.21ic.com/more.asp?name=iC921&id=9466" target=_blank>http://blog.21ic.com/more.asp?name=iC921&id=9466</a><br /><br /><br /><table class=ubb cellspacing=0><TR><td class=ubb>［035］</td><td class=ubb><br />［035］</td></TR><TR><td class=ubb><br />Simple&nbsp;charts&nbsp;are&nbsp;used&nbsp;to&nbsp;help&nbsp;the&nbsp;user&nbsp;select&nbsp;the&nbsp;proper&nbsp;cable&nbsp;without&nbsp;any&nbsp;technical&nbsp;understanding&nbsp;of&nbsp;the&nbsp;cable&nbsp;parameters.&nbsp;This&nbsp;program&nbsp;divides&nbsp;the&nbsp;usage&nbsp;categories&nbsp;into&nbsp;EIA-232-D,&nbsp;EIA-422-A,&nbsp;and&nbsp;EIA-423-A.&nbsp;When&nbsp;using&nbsp;this&nbsp;literature,&nbsp;use&nbsp;the&nbsp;EIA-422-A&nbsp;charts&nbsp;for&nbsp;choosing&nbsp;RS-485&nbsp;cable.&nbsp;<br /></td><td class=ubb><br />简单的曲线图可以帮助用户不需要任何电缆参数和技术认识情况下选择性能适宜的电缆。This&nbsp;program按用途分成三类：EIA-232-D、&nbsp;EIA-422-A和EIA-423-A。用这些文献时，可用EIA-422-A的曲线图来选择RS-485电缆。</td></TR></table><br /><br /><img src="http://www.bb-elec.com/images/rs422an-fig31.gif"><br />Figure&nbsp;3.1:&nbsp;Data&nbsp;Signalling&nbsp;Rate&nbsp;versus&nbsp;Cable&nbsp;Length&nbsp;for&nbsp;Balanced&nbsp;Interface&nbsp;Using&nbsp;24&nbsp;AWGTwisted&nbsp;pair&nbsp;Cable<br /><br /><br /><img src="http://www.bb-elec.com/images/rs422an-fig32.gif"><br />Note&nbsp;1:&nbsp;Above&nbsp;attenuation&nbsp;data&nbsp;is&nbsp;supplied&nbsp;courtesy&nbsp;Belden&nbsp;Wire&nbsp;and&nbsp;Cable&nbsp;Company&nbsp;<br />Note&nbsp;2:&nbsp;Datalene&nbsp;is&nbsp;a&nbsp;registered&nbsp;trademark&nbsp;of&nbsp;Belden&nbsp;Wire&nbsp;and&nbsp;Cable&nbsp;Company.&nbsp;Datalene&nbsp;is&nbsp;a&nbsp;type&nbsp;of&nbsp;foamed&nbsp;cellular&nbsp;polyethylene&nbsp;insulation.&nbsp;<br />Figure&nbsp;3.2:&nbsp;Attenuation&nbsp;versus&nbsp;frequency&nbsp;for&nbsp;several&nbsp;Data&nbsp;Cable

  

实际不会超过150米，电缆的特性阻抗应在150Ω左右！