摘要：
What happens when the opposing end of a serial link disconnects, powers down, or becomes disabled? The results may surprise you.
For example, when a link disconnects, your receiver may be so sensitive that it picks up crosstalk from your own local transmitter. That "self-reception" can trick your system into thinking it is in communication with the far end of the link when actually it's just sending packets to itself!
Circuits that detect the disconnected, powered-down, or disabled conditions are called carrier-detection circuits. The simplest forms of these circuits rely on separate interface pins that run parallel to the data path. These pins inform the receiver about whether the far end is disconnected, powered down, or intentionally disabled in a simple, although pin-wasteful, manner.
If your circuit is dc-coupled with logic 0 and 1 defined as 0V and VCC, respectively, then carrier detection is as simple as looking to see whether you have lately received any ones. The dc-coupled approach is useful only on short links within a single chassis in which you expect little or no signal attenuation. On a link that incorporates any sort of automatic gain control (AGC), you will find the dc-coupled approach quite useless. Once a link disconnects, AGC just turns up the gain until your receiver hears nothing but white noise, at which point the recovered data stream includes a copious mixture of ones and zeros.
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