Re: Troubleshooting X/Lisa RAM boards - first draft

>In the first three cases, my faulty memory board was installed in
>MEM1, with a known good board installed in MEM2. The first two
>boards would prevent the machine from booting if installed in MEM2.
>In the last case, I simply swapped the board's positions. (bad board
>MEM2, good board MEM1)
>
>> It's great to have more data!
>
>Yeah! One datapoint does not a study make :D

Your data certainly has me baffled. I'm obviously wrong about something. Perhaps the data at $186 is (sometimes?) MEM 2 then MEM1. eg. maybe the ROM changes the order to get working low memory and a working screen buffer.

Hopefully we'll get more people sharing their results.

>I'm really intrigued about the one board that came up all 00FF's. I
>wonder if one of the '373's is bad or something.

Yes, there aren't all that many problems that will make one byte bad.

The power for the low byte is filtered through L2, so you could check that for continuity, or check for +5 on pin 8 of the low byte RAM chips - columns 15-22 (Note the power pins on the RAM chips are the opposite from 74 series chips).

The 373 for the lower byte is at 13E; the enables are directly wired to the high byte, so pins 1 and 11 should connect to the same pins of the 373 at 12E.

If you have a logic probe, perhaps check for a signal at Resistor R1, which drives the R/W line for the low byte -- pin 3 of the RAM chips in columns 15-22.

And check finger 6 of the card edge for a dirty/bad contact.. that's the CPU low byte data select.

-- 
-----
You received this message because you are a member of the LisaList group.
The group FAQ is at http://lowendmac.com/lists/lisa.html
To post to this group, send email to lisalist_at_email.domain.hidden
To leave this group, send email to lisalist+unsubscribe_at_email.domain.hidden
For more options, visit this group at http://groups.google.com/group/lisalist