Device Select or Device Select bar(NOT)?

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vaf20

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in a bus-based system which one is better to choose any device ?
tnx ahead
 

usually it is taken active low(i.e device select bar)
 

hi my friend
yes i know , but my question is why active low is suitable ?
is there any protected scheme?
tnx ahead
 

Active-low mainly come from TTL days.

It is easy to use pull-ups resistors, and TTL, when having floating input, also saw a logic 1. So, when having, for example, a 3-state output, or an input going to a connector, it is possible that, when no output is driving a line, or when a cable is removed, that the chip input is left floating (ou pulled-up).

In this case, you would want control signals to not be asserted. So, this is why the standard is active-low (or inactive high). Because for TTL chip, it was easy to leave the input floating, which did let the input see a logic 1. Note that floating inputs only apply to TTL. CMOS inputs must not be left floating.
 

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