Character ROM / Video Problem - Dick Smith System 80

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Character ROM / Video Problem - Dick Smith System 80

Postby 3pcedev on Sat Jul 11, 2015 7:11 pm

Hi all,

I received my System 80 yesterday and have spent most of the morning opening it up and cleaning/checking everything. I also ended up having to build a video cable for it (forgot it used a DIN rather than RCA plug!)

Now I have run into a problem. When you power it up it displays "REEDY>" and then a heap of dollar ($) signs around the screen. The dollar signs are actually in a neat series of rows which I assume points to either a video ram or a character generator problem. Typing on the keyboard results in characters being put onto the screen, but not always the one you type. For example I might type 'A" and get 'F" or I might type 'G' and get 'G".

I then touchtyped in a simple basic program that loops and counts from 1 to 9. The program executes fine, however you dont get 123456789 - more like 12gh58789.

EDIT: Added more photos. It appears that it has some extra video RAM that has been piggybacked. Pictures tell the story there.

EDIT2: Should also mention I have reseated all RAM and socketed IC's on the boards.

Do you think its video ram or the character rom or something else? I have an eeprom burner so I can program a character rom (however I need the image for it).
Last edited by 3pcedev on Sat Jul 11, 2015 7:42 pm, edited 1 time in total.
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby 3pcedev on Sat Jul 11, 2015 7:23 pm

Here are some photos. First one is when you 'clear' the screen. Second is a LIST of the program I wrote, third is the execution.

https://plus.google.com/photos/10454057 ... pqG5oe3DFg
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby Jman on Sun Jul 12, 2015 8:09 am

HI
That looks like bad video ram. I had a similar issue on a Model 1.
If you have a scope look at pins 11 and 12 of the video ram chips 11 is the data in 12 is data out.
Compare pins 11 and 12 on the various 2102's and with a little luck you will see the odd one

BTW could you make a dump of your character generator and flick it my way ?

Regards
John
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby 3pcedev on Sun Jul 12, 2015 8:46 pm

Hi John,

Yes it did turn out to be bad RAM - Chip 3 is faulty. I swapped chips around until I the $$ changed to !!. Just got to get another IC and I should be back in business.

As for making a dump of the character generator I would be happy to; however it seems the 52116 isn't really a ROM; its an ASIC. If you have an idea on how to do it let me know; I'll see what I can do.
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby Jman on Mon Jul 13, 2015 9:24 pm

Hi
Fantastic news on the repair.
I don't know how to dump the character generator but I know it can be done.
Many years ago I had a character generator that was based on an eprom.
I shall continue the search

John
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby Audronic on Wed Jul 15, 2015 2:32 pm

@ Jman
@Tezza

Some System-80 Character Rom Ramblings.

https://www.dropbox.com/s/kh9hddy209byf ... 3.zip?dl=0

Hope that this is of some help.

Ray (Across the Ditch)
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby Jman on Fri Jul 17, 2015 8:03 pm

@Ray

Great info thank you.
Do you still have the PCB so we can map it out ?

Jman
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby Audronic on Sat Jul 18, 2015 11:21 am

@Jman

Sorry no, It went back to Ian Mavric (Trs-80) in Australia
http://ianmav.customer.netspace.net.au/trs80/


Good luck on your search

Ray (Across the Ditch)
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Re: Character ROM / Video Problem - Dick Smith System 80

Postby tezza on Sun Jul 19, 2015 10:13 pm

Thanks for those character generator resources Ray. Much obliged!
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