The VIC Chip and 6502 Processor drive/read/write the Address and Data bus on alternate cycles of the 6502-clock signal. So those 74LS245's are used for isolating. And you will see from the schematic that S02 is used to enable the 245's when the 6502 needs to drive that bus, SO2 been the 6502 clock. On the isolated bus, you have the 5K RAM and 1K colour RAM, the 6502 can read/write, and the VIC only reads. Given that your VIC Computer can execute a LOAD command, and given that the interrupt signal is generated, means that the kernel is up and running and also means the RAM is working, as the kernel has been able to boot far enough to be able to execute commands entered from the keyboard. Given the VIC Chip generates the clock, also means it is not necessarily bad. Can try probing the CS and RW signals that are connected to the VIC chip. It may be handy to connect (hook-up) a reset switch to your board, so you can easily reset the computer when observing a response on line you are probing, as the VIC chip is only written to a few time when it is been setup.
On the schematic, it starts at Pin 35 of the VIC chip as S01, then is inverted and that signal is called S02. But the main thing is that the VIC Chip and 6502 access the video memory on alternate phases of the SO2 clock (or So1 clock if one prefers).
I missed that bus sharing TBH, but it makes perfect sense and gives me another avenue to explore. Checking the CS & RW at the VIC is something I'll do once I get it back on the bench again (got an Apple "product" on there at the moment while I wait for some bits). Thanks for the suggestions - very much appreciated!
If you remove the 4066 chip and associated RAM UD2 , the computer will still work, expect the colours displayed will not be correct. If the RAM UD2 was not working, it should not effect what the VIC chips does.
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The VIC Chip and 6502 Processor drive/read/write the Address and Data bus on alternate cycles of the 6502-clock signal. So those 74LS245's are used for isolating. And you will see from the schematic that S02 is used to enable the 245's when the 6502 needs to drive that bus, SO2 been the 6502 clock. On the isolated bus, you have the 5K RAM and 1K colour RAM, the 6502 can read/write, and the VIC only reads. Given that your VIC Computer can execute a LOAD command, and given that the interrupt signal is generated, means that the kernel is up and running and also means the RAM is working, as the kernel has been able to boot far enough to be able to execute commands entered from the keyboard. Given the VIC Chip generates the clock, also means it is not necessarily bad. Can try probing the CS and RW signals that are connected to the VIC chip. It may be handy to connect (hook-up) a reset switch to your board, so you can easily reset the computer when observing a response on line you are probing, as the VIC chip is only written to a few time when it is been setup.
On the schematic, it starts at Pin 35 of the VIC chip as S01, then is inverted and that signal is called S02. But the main thing is that the VIC Chip and 6502 access the video memory on alternate phases of the SO2 clock (or So1 clock if one prefers).
I missed that bus sharing TBH, but it makes perfect sense and gives me another avenue to explore. Checking the CS & RW at the VIC is something I'll do once I get it back on the bench again (got an Apple "product" on there at the moment while I wait for some bits). Thanks for the suggestions - very much appreciated!
If you remove the 4066 chip and associated RAM UD2 , the computer will still work, expect the colours displayed will not be correct. If the RAM UD2 was not working, it should not effect what the VIC chips does.
Does the VIC 20 have a PLA chip like the C-64...
No, although it does have the VIC chip which is a custom Commodore IC that does the video, sound, and some processing of the system clock.