Re: CBM720 heads up

Hi there, some update and a question...

I've replaced three dRAM chips, and the CIA used for the IEEE488 and now the 
machine is working (even SID works). I'll update the pictures on Flickr when I 
have more time.

However, I'm looking for a working program to test the RS232 interface. I 
found the (two) CBM TERM program(s) - but unfortunately I have no idea how 
they work.

Has anyone some more information on these terminal programs (e.g. how to 
change baud rate etc), or another, more simple, terminal program for the 
600/700 machines?

Many thanks
André

On Sunday 23 November 2014 18:21:42 you wrote:
> Hi there,
> 
> On Sunday 02 November 2014 02:03:55 you wrote:
> > > Unfortunately IEEE488 does not work (at least with my Arduino XD2031
> > > setup... which probably doesn't mean much)
> > 
> > Check the CIA and TPI chips then.
> 
> Unfortunately I found that it didn't even get to using the IEEE bus because
> of memory problems.
> 
> It looked as if in Bank 1 Bit D5 would be flakey. Writing various patterns
> to it suggested that when you write a 0 to it, it would not be stable.
> Address patterns are located in various 256 byte pages of bank 1, but there
> in the top areas under addresses 127 and 255 of each page.
> 
> Unfortunately replacing the corresponding memory chip did not help - same
> error pattern with a new chip.
> 
> So what do you think it could be?
> - "bank 1" is NOT what uses "/CASSEG1,RASSEG1" in the schematics? then I
> have replaced the wrong chip (from the wrong bank)
> - I think the data bus buffer U33 is highly unlikely. It buffers all DRAM
> accesses and the error is specifically located
> - One of the four 4-to-1 selectors U27,U28,U34,U35? Bit patterns of the
> error would then suggest multiple of those
> - MUX decoding for those selectors? would that not influence all addresses?
> - Or the PLA U75 in the end which is supposedly known to fail? It creates
> /CASSEG1,/RASSEG1. But why then only some of the addresses within the bank?
> 
> I'm baffled.
> 
> https://www.flickr.com/photos/afachat/sets/72157647798386959/
> 
> (see also
> http://www.vintage-computer.com/vcforum/showthread.php?45083-Commodore-B720
> -repair )
> 
> Any help appreciated!
> André
> 
> 
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