Logic Analysers

[Adrian Graham]

On 02/02/2017 03:13, "Jon Elson" <elson at pico-systems.com> wrote:

> 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.

True, but one is adjustable for either 3.3v or 5v threshold while the other
is fixed 5v. Also you'd expect that sampling at four times the clock speed
(they'll both do 25Mhz with 6 channels) then every pulse would be picked up.
 
> Generally, you would clock the LA off some master clock in
> the unit under test.

Yeah, neither of them will do that. Units with external clock capability are
over the ukp100 mark which initially was far more than I was comfortable
paying when I wasn't sure about how it all worked. For troubleshooting CBM
PETs the cheapy one worked fine.

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?