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the thing is, data out from test is decared as an OUT so tri-stating it is useless, as nothing can drive into it.
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Honestly, I'wasn't sure about the effect of defining dataout as output but tristating it. Apparently the compiler doesn't complain about. But I never use out type for a tristate port, always inout.
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if I set dataout as '0' when 'sel' is '0' or keep the value as same as before.The compiler will give errors: can't be assigned more than one value.
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This will surely happen, if the connecting signal data_reg is driven otherwise. But it isn't in the code you have presented. So you are apparently talking about different code.
That's the problem when presenting only snippets for the discussion. People can only refer to things, that are shown within. If you have any open questions related to your code, please provide a full example, that reproduces the problem.
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