Am 16.08.2016 um 23:00 schrieb Antonio Carlini:
This was when I was in DEC so with a bit of hunting I found the description of the problem. IIRC there was an issue with the 74LS243 bus transceivers.
Interesting. I evaluated parts suitable as Q-Bus drivers/receivers recently and noticed that for example a TI SN74S38 will quickly reach extreme values for high-Z leakage current (I_OH in the datasheet) when warmed with a heat gun (and become out-of-spec for Q-Bus of course)... Values are even quite different for ICs of the same batch.
It could be a similar issue with specific batches of 74LS243, i. e. they will impose an unconsidered load (when the machine was designed) for other participants on the (memory) bus when it gets warm.
Especially leakage current can be extremely dependent on temperature. Felix