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Basic logic analyzer for recording slow aperiodic signals?

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ds18s20

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Hi,

I have been looking for a way to record traces of 1W waveform. It is no different than a ttl-level rs232 with bit time of about 100uS which is no different than 9600 baud serial communication. The problem is that I don’t have digital scope and really, for something that slow I should be able to use a parallel port logic analyzer

The problem is that the logic analyzers that I have looked at all want to have their inputs clocked by an external clock source. That is understandable for the most common use of logic analyzer but in my case I want to use a logic analyzer much like one would use a digital oscilloscope. I want the analyzer to sample the input at specific time interval which is generated internally. So in my case, I expect a bit time of 100uS so the logic analyzer would need to read its input at a 50uS (the classic DSP sampling formula of f/2) rate which is about 20kHz

Can this be accomplished at all with a simple parallel port logic analyzer?
 

All the logic analyzers that I've seen have two acquisition modes: state and timing. State mode requires a clock from your project. Timing mode uses a free-running internal clock, like an oscilloscope.

The PC's parallel port doesn't have a timer to give you a predictable sample rate. Operating system interrupts and other activity will make things worse. If I was stranded on a desert island with only a PC and parallel port, I suppose I could write a (DOS?) program that disables interrupts and then spins in a loop reading the parallel port along with time-stamp values from the Pentium's RDTSC opcode (it reads the CPU's clock cycle counter). That way I'll know approximately when the samples were acquired. The sample rate would be roughly 500kHz to 1MHz.

Search Google for "parallel port logic analyzer" and you will find several projects like this one. I wonder how it measures time.
https://www.xs4all.nl/~jwasys/old/diy2.html

That seems somewhat painful. Having a real scope or logic analyzer would be much nicer.
 

Re: Basic logic analyzer for recording slow aperiodic signal

Thanks, yes I did find several and I downloaded the most credible-sounding software and although it does work, it needs external clock. Needless to say the results suck. I know that having a real analyzer would be better but I just dont have one.

I agree, the int-plauged multi tasking OSes are worst when it comes to timing, I was hoping that given the slow speeds of this project it might just work but.. ehhh

~B
 

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