epm7128的问题？？？？

winterred

大家好！
       在我使用7128时，发现有两个引脚90-OE1和92-OE2的用法不太明白；手册上说All I/O pins have a tri-state
buffer that is individually controlled by one of the global output enable
signals or directly connected to ground or VCC.好像是这两个接地或接高都可以开通三态，但是后面又说When the tri-state buffer control is connected to ground, the output is tri-stated (high impedance) and the I/O pin can be used as a dedicated
input. When the tri-state buffer control is connected to VCC, the output is enabled.我就迷糊啦当接地时，管脚可以作输入，难道不可以作输出？？接高就不可以做输入啦吗？？我在电路中接地啦，各个管脚都可以双向的，我不明白的是这个三态什末意思在这有何用？？？