The address could be stuck internally on the RAM so the address line would appear OK externally. The fact the ROM runs correctly with the shared Address line would indicate the address lines are generally OK. It could be floating on the module as you suggest, look to see if there is any corrosion on the module and the pins to the module are in good condition. Perhaps try reflowing each of the module pins to make sure there are no dry joints?
I am currently looking into producing some sort of test harness for the model T however the lack of socket for the 8085 makes it hard to add in a test fixture. Even trying to tri-state the 8085 appears difficult as it does not tri-state the ALE, all the other lines are OK. Another option would be to try and monitor what it does with something that plugs into the RAM socket. That way at least there would be a trace showing the bus activity. Being able to single step the 8085 might be another option with a known ROM in the socket. From: M100 <[email protected]<mailto:[email protected]>> on behalf of Jeffrey Birt <[email protected]<mailto:[email protected]>> Reply-To: <[email protected]<mailto:[email protected]>> Date: Thursday, May 10, 2018 at 7:13 AM To: <[email protected]<mailto:[email protected]>> Subject: Re: [M100] SPAM-LOW: Re: New member - question on 'half' alive Model 100 Very good observation. I believe that I did check that all address lines were toggling at the RAM module, but I will check again. Perhaps A4 is floating? Yes, a RAM test would be nice. I think a diagnostic ROM could be made to do that, just start out by testing the standard RAM (without any stack operations) There is a diagnostic cartridge for the C64 that does that, of course if the address decoding is not working then the results of such a test are bogus 😊 Thanks, Jeff
