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SiriusHardware · 28th Jun 2021, 5:14 pm

The 'Top Page' output needs to be available on more than one pin header because it may occasionally need to be linked to more than just one of the page select inputs - for example it may also be linked to graph / chars to change from one mode to the other half way through the screen and maybe even also to 'swap pages' depending on whether you want the graphics half to be on the upper half rather than the lower half. So right there is an example where you might need to have three jumpers or links going to Top Page.

I would also provide for a modest value series resistor in the 'Top Page' signal output from the chip because with the amount of patchability available it would be quite possible for Top Page (an output) to end up connected to a page select input which is shorted to GND via a closed switch, or to end up in opposition to an 8154 port pin which is set to be an output. That's an especially expensive mistake to make on a real SOC VDU but you don't want to make it too easy for Tim to fry all of his 877s, either.

28th Jun 2021, 5:14 pm	#9
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,553	Re: Ortonview PCB The 'Top Page' output needs to be available on more than one pin header because it may occasionally need to be linked to more than just one of the page select inputs - for example it may also be linked to graph / chars to change from one mode to the other half way through the screen and maybe even also to 'swap pages' depending on whether you want the graphics half to be on the upper half rather than the lower half. So right there is an example where you might need to have three jumpers or links going to Top Page. I would also provide for a modest value series resistor in the 'Top Page' signal output from the chip because with the amount of patchability available it would be quite possible for Top Page (an output) to end up connected to a page select input which is shorted to GND via a closed switch, or to end up in opposition to an 8154 port pin which is set to be an output. That's an especially expensive mistake to make on a real SOC VDU but you don't want to make it too easy for Tim to fry all of his 877s, either.