When writing programs to output data to a printer port, on a vintage 5155 computer, if I set up the simplest possible program loop to set the port data bits high & then low & loop. The output waveform has a low frequency timing irregularity (phase jitter) of perhaps a few hertz. And this jitter seems about the same interval, regardless of how fast the program executes. For example I have attached a scope images of it running in usual GW-Basic, very slow being an interpreted language and running in compiled Quickbasic which is much faster. But the phase irregularity (disturbance) in the signal timing is the same in both cases. Probably a video of the scope screen would have been better.
It is as though at a low frequency interval the processor is being briefly interrupted from its simple program loop by an I/O device, or could this be the consequence of electro-mechanical jitter from the hard drive ? Does anyone know, or can guess at, the origin of this timing disturbance ?
(This phase jitter would be of no consequence for data exchange etc, but can be for another application where the signals were used for another purpose).
It is as though at a low frequency interval the processor is being briefly interrupted from its simple program loop by an I/O device, or could this be the consequence of electro-mechanical jitter from the hard drive ? Does anyone know, or can guess at, the origin of this timing disturbance ?
(This phase jitter would be of no consequence for data exchange etc, but can be for another application where the signals were used for another purpose).