Tuesday, August 7, 2012

XEGS Success!

Those of you following this blog early on may have noticed that I abandoned the Atari XEGS system as a platform for the Liber809 due to an issue that I was having with getting the system working in All-RAM mode.  My initial suspicion was that there was some subtle timing issue with the Liber809 board that caused it to fail in the XEGS.  Any write to $BXXX would cause what I *thought* was a write to $DXXX.

In actuality, writes to $BXXX were going to $FXXX and blowing away the bitmap font and display list that were setup there.  I confirmed this, then asked the folks on atariage.com for some assistance.  After looking at the schematic, it became apparent that either the two 4464 DRAMs or the FREDDIE chip were at fault.

I spent some time this evening desoldering and socketing the two 4464s, then replaced them with known working extras that I had.  After hooking everything up, the problem still persisted.  Any attempt to write to $BXXX was actually going to $FXXX.  That left the FREDDIE as the possible culprit.  All address lines go into that chip, and it's responsible for reading/writing/refreshing the DRAMs.  It seemed like the next logical step.

Since I don't have spare chips around, I turned to my trusty 130XE which had its FREDDIE soldered onto the board.  I carefully desoldered the chip and removed it gently, then inserted it into the already socketed FREDDIE location on the XEGS and started it up... SUCCESS!  The XEGS did not have any problems booting with the Liber809 ROM and 6809 processor.  NitrOS-9 came right up and I was able to interact just fine.

So it appears the FREDDIE that was in the XEGS was "sticking" A14 somehow.  Now to find a replacement FREDDIE.

On another note, my 130XE keyboard mylar was failing.  Keys were simply not working.  I found a supplier of replacement mylar keyboards, http://www.best-electronics-ca.com, and sent an email.  They were very prompt in responding, and I even picked up replacement key cups for both the 130E and XEGS keyboards, which eliminate the "spongy" feel.  I ordered on a Friday and the parts were here on Monday.  I then installed the keyboard mylar and key cups, and boy did it make a difference!  Check these guys out.  They are EXCELLENT Atari product suppliers!