The best way to inspect digital signals is with a logic analyser. Sometimes it's useful to be able to do this on-site, or you may have to take a ‘floating’ measurement. The four-channel logic analyser described here is suitable in both situations due to its compactness and because it can be battery powered. The maximum sampling rate is 2 MHz and the circuit has sufficient memory to store 1024 samples of the signal. The dot-matrix display with a resolution of 64 by 128 pixels shows a clear representation of the digital signals.
The circuit diagram erroneously shows X1 as a 20 MHz crystal. This should be 10 MHz as stated in the parts list.