The ‘oddness’ is due to the buffers tri-stating or entering a high-impedance state, as the data-sheet calls it, when ALE goes high. If you put up both the address and ALE trace they should match. If the inputs and outputs of the buffers look good then the next step would to check the address decoders to make sure the ROM, SRAM and IO ports are being selected. The ROM CS should go low immediately after the reset goes high to fetch the 1st instruction from 0000H. For the M100 the first instruction is a JMP 7D33H - the boot routines.
There is an M100 ROM disassembly somewhere but I cannot find the link to it at the moment. That I think would help a lot with the debug process. Not sure what type of rosin they used for the soldering process but I have being using 90% CVS isopropyl alcohol to remove it without much problem. 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: Wednesday, May 2, 2018 at 3:25 AM To: <[email protected]<mailto:[email protected]>> Subject: Re: [M100] SPAM-LOW: Re: New member - question on 'half' alive Model 100 Thanks for the response concerning the 40H parts. I took some time this evening to very carefully look at all the signals on the 80C55. I thought I saw some odd waveforms and after setting the scope to single shot mode I did indeed see some oddness on AD0 through AD7. Take a look: https://1drv.ms/u/s!AtH4vpaZnzX7j8A29GFkjLbQlWpLUA<https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2F1drv.ms%2Fu%2Fs!AtH4vpaZnzX7j8A29GFkjLbQlWpLUA&data=02%7C01%7C%7Cc9a81e6722d9409f239208d5afd40610%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636608247519969338&sdata=79DwUiP8IQhmbwYzZCf0tIcl6bSU0GRMz381YCR72MQ%3D&reserved=0> I checked that it is the same at the 80C55 side of M1 and M2 while the ‘other’ side of M1 and M2 has nice square waves. I then checked that the control signals ALE and /RD appear to be correct and they do. This leads me to believe that one or both of M1/M2 are defective. I believe I have a 74HC version of these on hand. I very much appreciate being steered in the right direction 😊 On an unrelated note: what kind of evil flux did they use on these boards. I tried to clean the flux off of my capacitor installation and wound up taking 30+ minutes to flush the back side of the PCB with alcohol several times to get the yellow tinged flux off. It wanted to form a nasty white chalk type coating. Thank again, Jeff
