Bryan Parkoff wrote:
"David Wilson" <mcs6502@gmail.com> wrote in message 1159751125.836906.154290@b28g2000cwb.googlegroups.com">news:1159751125.836906.154290@b28g2000cwb.googlegroups.com...Bryan Parkoff wrote:Okay, lets say if ALTVID' has low signal. Will the screen be invertedincluding inverted 16 colors?David,No. As I said - if ALTVID' is low, the video will be high (white if ALTVID' is low for at least 2/7 of a microsecond).Will it show white border in the top, bottom, left, and right margin?The margins are normally black due to the signal WINDW' (aux-7) from the IOU chip driving OE' on the chargen. As ALTVID' is connected after the LS166 parallel->serial chip this means that ALTVID' can even override the black margins. So yes, you can use ALTVID' to make the margin white.I understand. It makes sense when NAND gives 4 possible data 0, 1, 1, and 1. If ALTVID is high, it allows LS166 to flow serial bit stream through an inverter '0' to '1' and '1' to '0' so '0' and '1' is used for output of NAND. If ALTVID is low, the output of NAND is '1' and '1'. I agree that the background is solid white if ALTVID is low. My question is -- would the border be white? You claim to be black border with white background, but I claim white border and white background. I agree that WNDW is high to disable VIDEO ROM chip to stop flowing 8 bit parallel data to LS166 chip however load/shift register is still loaded with "11111111" or 0xFF during high WNDW. Then "11111111" is shifted out through inverter inside NAND to be "00000000" so border is black during blanking. If you replace NAND chip to inverter chip while it ignores low/high ALTVID, the result is the same as before. If you remove NAND chip and allow LS166 chip to be shifted out to the video signal directly without through NAND / inverter, you will see white border and inverted 16 colors. Does it make sense? Why is ALTVID needed? I guess that if RGB card is installed in aux card to block the users from viewing NTSC monitor. I use RGB card, but it allows me to view RGB monitor and NTSC monitor at the same time.
As the name suggests, /ALTVID is there to enable video to be generated not by the main board, but by alternative logic. -michael New, faster SUDOKU v2.0 solver for Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."