Timer

I must interrogate the serial port once per second so I used
the TIMER CONTROL. (1000 msec.)

This control is not accurate, I don't know why.
How do I upgrade the time interval accuracy ?

Thank you.

Use the QueryPerformanceCounter and QueryPerformanceFrequency APIs.

I think the lowest interval of an object timer (the general control) is about 55 milliseconds. What was the interval set at?

The lowest interval is 1ms, but that is only accurate to about 55ms, because of the way the Timer control works internally. The APIs i mentioned above are (AFAIK) as good as it gets. There have been many threads about this though, search for "performance" or "timer"

If you are simply looking for a timer that has an accurate 1ms
resolution (QueryPerformance APIs are much more accurate
than that, but more complicated to use), then I believe timeGetTime
would be sufficient for your needs.

If you are making some sort of absolute graphing program or a
stopwatch or something, you would need QueryPerformaceCounter,
but for general timer stuff, it's not needed.

I have this code to do a thermometric curve length 2 hrs

Private Sub Timer1_Timer ()

MsComm1.Enabble = True
......
code
.....
End Sub

There is a time error of 20%. I suspect that the time interval is not real, but is the sum of the interval and an unknown time necessary to elaborate the code. I don't want this.
I ask you: How do i connect the code direct to the system watch ?

Use a Do While loop, in conjunction with one of the APIs above.

Do
tempVar = timeGetTime
'... code ...'
Do Until timeGetTime > tempVar
'do nothing
Loop
Loop Until bEndSomething like that.