Logic Analysers

[Jon Elson]

On 02/01/2017 05:18 PM, Adrian Graham wrote:
> Evening folks,
>
> I have two so-called Logic Analysers, both cheap Chinese clones of other
> more expensive units that hook up to the host via USB2 and stream readings
> direct to software, in one case the open source Sigrok and in the other
> genuine Saleae Logic.
>
> I'm getting different and inconsistent readings out of both of them whilst
> sampling at 25MHz which should be more than enough for this 6MHz Executel
> I'm working on. Both of them are good for spotting dead or stuck outputs but
> I still can't get a good set of readings from eg all points on a single
> address line. Tonight I replaced all four ROM sockets and ROM chips, tested
> each individual line for resistance (0.5 ohms on all apart from an
> occasional 0.4 and 0.6) but still get ghost readings.
>
> Is it me or the cheap clones?
Well, these lines likely have pulses, and depending on when 
they are sampled, you can get a high, low, or indeterminate 
state.  Also, the LA may have fixed threshold voltages that 
may not correspond to the actual thresholds of the logic you 
are testing.

Generally, you would clock the LA off some master clock in 
the unit under test.

Jon