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Character Rom Replacement for a Vic-20 ??
Posted: Tue Jul 11, 2023 3:04 am
by audronic1
Hi All
Have been making up a Character Rom Replacement for a Vic-20
Using a 2764 Eprom. But I am unable to get it to work
If I Use a 27C32 eprom this works. Is this normal
What do most people use ??
Thanks
Keep Safe
Ray
Re: Character Rom Replacement for a Vic-20?
Posted: Tue Jul 11, 2023 3:53 am
by Mike
If you perhaps add a schematic of how you wired the pins of the 2764 that would help to assess what probably went wrong.
The mainboard uses VA12=VA13=0 for the two chip selects of the original character ROM, mapping those to /CS and /OE of a 2764 works the same way as with a 2732, regardless whether they are NMOS or CMOS. On the '64, for the ease of use, you can burn the character set twice to $0000..$0FFF and $1000..$1FFF. It then doesn't matter whether A12 on the chip is tied to low or high level, but A12 should be defined nonetheless.
Other than that, a sharp look on the speed grade of the EPROM is in order. It should have an access time of 300 ns or less. 400 ns likely is too slow.
audronic1 wrote:What do most people use?
I can't give a first-hand statistics, but I'd suppose people just grab and use what's in the spare part box.
Re: Character Rom Replacement for a Vic-20 ??
Posted: Tue Jul 11, 2023 5:12 am
by Lechuck
Hi Audronic,
I did it using the diagram I attached to your other post and worked ok for me. Have you used the same diagram?.
Re: Character Rom Replacement for a Vic-20?
Posted: Tue Jul 11, 2023 6:38 am
by audronic1
Mike wrote: ↑Tue Jul 11, 2023 3:53 am
If you perhaps add a schematic of how you wired the pins of the 2764 that would help to assess what probably went wrong.
The mainboard uses VA12=VA13=0 for the two chip selects of the original character ROM, mapping those to /CS and /OE of a 2764 works the same way as with a 2732, regardless whether they are NMOS or CMOS. On the '64, for the ease of use, you can burn the character set twice to $0000..$0FFF and $1000..$1FFF. It then doesn't matter whether A12 on the chip is tied to low or high level, but A12 should be defined nonetheless.
Other than that, a sharp look on the speed grade of the EPROM is in order. It should have an access time of 300 ns or less. 400 ns likely is too slow.
audronic1 wrote:What do most people use?
I can't give a first-hand statistics, but I'd suppose people just grab and use what's in the spare part box.
Hi Mike
Ok I have used a " VIC-20 CHR ROM REPLACEMENT" PCB which is known to work
I will have a look at what A12 is doing ??
And the Speed of the 2764
Yes I have Burned the Code Twice.
I was mainly curious as to If Both types worked.
Thanks
Keep Safe
Ray
Re: Character Rom Replacement for a Vic-20 ??
Posted: Tue Jul 11, 2023 6:53 am
by audronic1
Lechuck wrote: ↑Tue Jul 11, 2023 5:12 am
Hi Audronic,
I did it using the diagram I attached to your other post and worked ok for me. Have you used the same diagram?.
Hi Lechuck
I have used a " VIC-20 CHR ROM REPLACEMENT" PCB which is known to work
Keep Safe
Ray
Re: Character Rom Replacement for a Vic-20?
Posted: Fri Jul 14, 2023 1:29 am
by Mike
audronic1 wrote:known to work
How are the 3 jumpers and the pin arrangement in the top half (seemingly 4 DIP switches) connected?
Re: Character Rom Replacement for a Vic-20?
Posted: Fri Jul 14, 2023 5:04 am
by JonBrawn
Mike wrote: ↑Fri Jul 14, 2023 1:29 am
audronic1 wrote:known to work
How are the 3 jumpers and the pin arrangement in the top half (seemingly 4 DIP switches) connected?
Those 4 things are more likely resistors, I reckon, rather than DIP switches.