Propagation delay

Full-/high Speed: ≦26ns/cable

 Low speed: 18ns /cable

大家见过此标准的分类吗

非常感谢

我查到了

我有点不理解

为什么Low speed: 18ns /cable反而比Full-/high Speed: ≦26ns/cable要求的还要严格呢

這點我也比較困惑

其它哪位大侠可以帮忙解惑

甚是感谢！

roger呢？

哪位大虾帮帮忙吧

急呀

客人急需我的回复

我自己却一头雾水

怎么交得了差啊

先谢了！

标准啊。

HDMI不也有这样的现象吗？

HDMI标准中的哪个现象与此类似

楼上的朋友能不能解释的更清楚点呢

谢谢！

谁能帮忙解释一下啊

各路高手不可能都不知道吧

希望不吝赐教

此问题始终悬而未解

再次期待中......

还是李小龙大侠热心肠

多谢

嗯,full-speed是USB1.1的高速模式，high-speed 是USB2.0模式,full-speed并非全速的意思，它是相对high-speed而言的，实际速度为12Mbps，与USB1.1完全一样，所谓得full-speed其实就是USB2.0对USB1.1的兼容模式，USB2.0 （full speed）不等于USB2.0 full speed，它真正得含义是USB2.0工作在USB1.1 full speed模式下。

会不会是full speed模式其实是USB1.1标准里面,所以会要求没Low speed的严格?我这样理解的,不知对不对?

［卓一航 在 2009-7-6 17:57:22 编辑过］

［卓一航 在 2009-7-6 17:58:11 编辑过］

USB传输速率划分为

LOW SPEED

FULL SPEED

HIGH SPEED

SUPER SPEED

照常理解，都应该是速度的提升，各特性参数应该会更严格

我唯一的想法

觉得可能是由于当时PC端电子设备技术的局限性

为能达到USB的设计传输速率

从而限制整个线材的用线长度

故将Delay值规定较小

嗯,也只能这样解释.....,或许以后USB协会芯片更先进了,以后的延时要求越来越松,线材使用也可以越来越长了.

colthuang:

嗯,也只能这样解释.....,或许以后USB协会芯片更先进了,以后的延时要求越来越松,线材使用也可以越来越长了.

这也只是我的个人推理

我也不知道这样是不是合理的解释

我想应该有更合理的逻辑

aou83:

Delay is measured from the Series A connector pins to the Series B connector pins and is no more than 26 ns. For other cables, the delay is measured from the series A connector to the point where the cable is connected to the device. The cable delay must also be less than 5.2 ns per meter.

请问这里的5.2 ns per meter，为什么定义这个5.2ns/m值？和介电常数有关系吗？

chiiwa:

请问这里的5.2 ns per meter，为什么定义这个5.2ns/m值？和介电常数有关系吗？

我也不清楚，是規格定下的

There are two types of cables: 

1. Low-speed cables
2. Full and High-speed cables 

All low-speed devices must use a “captive” cable assembly as defined in Section 6.4.3 of the USB 2.0 Specification.  Common examples of low-speed devices are keyboards, mice and joysticks.  This requirement forces the cable to be specific to the device.  In other words, a low-speed device will have its own specific custom cable. 

Full-speed devices and high-speed devices use the same cable, called a standard detachable cable assembly.  Please see Section 6.4.1 of the USB 2.0 Specification.  The standard detachable cable assembly can be used by any full-speed or high-speed device that uses a USB B-receptacle.  Standard detachable cable assemblies can be purchased at stores.  

Full-speed and high-speed devices can have a captive cable.  There is no differences, electrically, between standard detachable cable assemblies and a captive cable of a full-speed or high-speed device.  A common example of full-speed and high-speed devices that use captive cables are webcams (web cameras). 

To reduce consumer confusion on cables, only full-/high-speed detachable cables are permitted.  Low speed devices are not allowed to use standard detachable cables.  Instead, they must have a permanently attached cable or use a custom connector. 

Section 7.1.1.2 of the USB 2.0 Specification explains why low-speed captive cables have to be shorter.  The capacitive load of long low-speed cables differs significantly from full- and high-speed cables.  This causes large signal reflections in the cables of low-speed devices.  The shorter cable minimizes the reflections.

樓上的意思是low speed 的delay比high speed嚴是因爲長度比較短嗎？