Hello.I'm trying to build my own Apple 1.I have a question on the video terminal operation scheme.When it is fully assembled and working, "_ @" symbols are displayed on the monitor screen.According to the datasheet at 2513 you can see that the "@" symbol is generated when Low levels (0) are set on the address lines A4-A9,while displaying the "_" symbol, A4-A8 lines are set to High Levels (1).
Question: Where does High(1) come from on these lines? After all, according to the scheme (for example, the section with C4 chip), the outputs of the registers are pulled to ground (-5V)through a resistor and respectively at the input of the comparator 74157 will be LOW(0), which is transmitted to the input of the register 2504, which is looped filled with all zeros (although it was already filled with zeros when power was turned on).Signal WRITE is set to high, 74157 selects B input. CLR is change by counters, and when it is High, the comparator output is set to LOW, not High.
How High on lines A4-A8 appears - I can not understand.