[Techtoolslist] Noise…

[John Robertson]

I was trying to sort out a Rottendog System 9TO11 MPU that would work with some EPROMs - sometimes - but not others. 

So pulled out the 9010 and 6802 pod and did some signatures.

Sure enough the signatures were not the same ones twice.

Made a simple RAM test for verifying the Data and lower Address lines at the EPROM positions - used a 6265 RAM and tied the /WE line to pin 27 of the CMOS RAM (R/W). 

And the 9010 RAM tests worked just fine so I was sure the Data lines and lower Address lines (A0-A12) were good.

Scope on A13 and A14 were inconclusive.

Pulled out my 6802 NOP CPU (hard wired to NOP) and scoped A13 - lots of noise on the bottom half of the cycle!

Replaced the 74HCT244 buffer and the board is again a happy camper!

I do like my NOPs for checking Address lines! 

I made them years ago for the common 8-bit CPUs and use them periodically on weird problems that the Fluke 9010 can’t nail down. Just bend the CPU’s Data leads up and tie them as needed to produce the NOP code. 

John :-#)#