[ntp:questions] Performance?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Tue Dec 30 17:39:28 UTC 2008


Unruh wrote:

> Garmin defines the leading edge as the transition from 0V to 5V on the PPS
> line. Now serial has two levels -12V and +12V. with capacitive coupling,

There's no capacitive coupling; it is assumed that the transmission line 
  and load behave as capacitive, but that is in parallel.

The transition region for RS232C is -3V to +3V into 4 to 5k.

> the garmin signal would be something like -2.5V to 2.5 V which really is

However, the expected hysterisis is in the 10s to 100s of mV range, and 
the actual threshold for control signals is supposed to be above zero, 
such that zero volts will give an unambiguous off state.

In practice, therefore, with real world RS 232 line receivers, there is 
no problem in feeding from TTL levels, as long as the cable is short 
enough to avoid ringing.

> way out of spec for the serial port. (Or with the typicallysmall duty
> cycle, more like -1V to 4V) 




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