对了一下，电路图是没有错的。检查一下焊接，供电等等。同时，注意：
With auto-powerdown enabled, the device is activated automatically when a valid signal is applied to the receiver
input. The INVALID output notifies the user if an RS-232 signal is present at the receiver input. INVALID is high
(valid data) if the receiver input voltage is greater than 2.7 V or less than −2.7 V, or has been between −0.3 V
and 0.3 V for less than 30 µs. INVALID is low (invalid data) if the receiver input voltage is between −0.3 V and
0.3 V for more than 30 µs. Refer to Figure 5 for receiver input levels.
以下下面的两个真值表，弄不好没有输出是可能的。 http://www.ti.com/lit/ds/symlink/max3221.pdf
你可以通过INVALID 检查一个芯片这时是否已经工作。
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