"block B" : digital clock registers with display
 
 

address decoder

o	an exercise in decoding a unique binary code so that an action may be performed when such a code is detected
o	useful for a specific hardware module connected to a microprocessor address bus, so that this module is able to respond when it is addressed
o	the address being decoded can be easily changed through the use of external inverters to make this black box a general-purpose one

 
 
 
 
 

selector switch

o	an exercise in selecting which input to present to the output
o	this is in preparation for an understanding of the operation of the 74ls258, which has two 4-bit inputs to be, respectively, selected by the level ("1" or "0") that a selector pin is at

 
 
 
 
 

74LS258 selector

o	the 258 selects whether "Counter A" or "Counter B" is displayed
o	this can be used for viewing the time at a default selector pin position, or the alarm clock setting at the other selector pin position
o	going beyond the alarm clock application, this is also be used to bring external binary bits into the computer printer port via its five input pins
o	the cascading of selector chips for input data multiple bits is a "must" for computer interfacing projects