The author of the interface arrived on the forum and suggested a few more interesting arguments for using dedicated testers. I must say that I fully agree with it
Let me quote it in the original trojacek is right, it is not possible to check for correct timings, even very expensive testers with much more power as a ATmega2560 cannot do this perfectly.
There are several issues a "simple" tester cannot do:
- checking timings: you need a very high resolution of 1-5ns to check if a rising or falling flank is correct. Only very expensice equipment (e.g. from Fluke) can reach this resolution.
- correct level of signals: you have to use analog ports to check if a signal level is correct. You have to measure the signal levels during the testing and check if the TTL levels are correct.
What a simple tester can do:
- it can check of a memory cell is in principle working
- it can check if refreshing is working in principle
- it can check if a row or column line is not working correctly
These simple tests can detect 99.9% of defect memory chips for an affordable price. Maybe you will have a false positive (you will recognize when you use this chip) or a false negative (then you loose one chip) but you will still have false positives and negatives also with expensive testig equipment.
So affordable memory testers are a good compromise (or spending thousands of EUR for having maybe 0.5% more accurate tests?).
Testing in a real computer is never a good idea. Depending on the number of RAM chip you want to test, you have to switch the computer on and off several times that will stress the old hardware and maybe this will lead to other defects. Also mounting and dismounting chips several times will stress the old sockets. So not really a good idea.
I have tested hundreds of chips whith it now and will not longer miss it. Also the feature to identify (EP) ROMs is great. I have quite often ROMs that are not labeled, and it is really convenient to put these in the tester and see that is e.g. a "901447-05 / 39-03 (H3) / PET Basic 1/2 $ E0" ROM;)
And you can use the tester to create your own tests. The hardware ports are documented, so you can write also your own software if you want.
