Ramtest as monitor replacement shows faulty bits!

3 posts / 0 new
Last post
Offline
Last seen: 1 week 3 days ago
Joined: Feb 17 2024 - 06:58
Posts: 8
Ramtest as monitor replacement shows faulty bits!

To test my prom-development-adapter I wrote a RAMtest that runs as monitor replacement @FF00-FFFF.

It needs no ram (zeropage, stack) and tests page 0-2.

First faulty bits will be shown on screen as 1.

The test runs continously.

Here is it: https://github.com/vossi1/a1-ramtest

 

 

 

Offline
Last seen: 4 hours 5 min ago
Joined: Apr 1 2020 - 16:46
Posts: 1174
No damping resistors ?

See my post #13 in this thread:

 

https://www.applefritter.com/content/replace-2504-1404-am2804pc

 

... for the hints about the damping resistors (part of my "reliability mods", published an Applefritter years ago).

 

My rule is to put these resistors in first, and then see if the DRAM works. If not, check the timing with an oscilloscope. If the timing is right (the 480ns oneshot in particular) and the DRAM still does not work, add the extra bypass capacitors. And if that doesn't help, then you are at the start of a long debugging journey.

 

- Uncle Bernie

Offline
Last seen: 1 week 3 days ago
Joined: Feb 17 2024 - 06:58
Posts: 8
UncleBernie wrote:See my post
UncleBernie wrote:

See my post #13 in this thread:

https://www.applefritter.com/content/replace-2504-1404-am2804pc

 

I know the reliability mods with damping resistors and extra caps. Woz used really not enough caps. Normally you need a cap at every TTL and every supply voltage at the dram. Also floating inputs at TTL/LS are ok, but not good.

 

The LS board works with SN74LS257 and ITT dram perfectly with Mike Willegal's RAMtest without any mods.

It has with a real 22k resistor (not AB) about 440ns - but that works nice with ITT rams. But later I use my RAM/ROM card and no onboard dram ...

 

Also my real TTL board runs with mostek-rams without any mods/extra caps over hours.

I measured at this board the 480ns and with 22k/12k from AB its ok. The Allen bradley 22k has in reality 25k and the 12k has about 13k - and thats good.

Christian

Log in or register to post comments