Elenco MicroMaster ® Computer Training Kit - Manual (Page 21)

3-5
This switch connected to the 8155 integrated circuit chip enable (CE) pin through transistor Q3. When the
switch is down (toward the edge of the board) the transistor is turned off. The CE pin is held high by resistor
R69. In this state the 8155 is disabled and will ignore input data on all of its other pins. When the switch is
up, the transistor turns on and the CE pin is held near ground. In this state, the 8155 is enabled and will
respond to inputs on the other pins. In a later lesson, the reset switch will be used to allow address line
A15 to control the CE input.