[ntp:questions] Garmin GPS 18LVC Setup but questions on best way

David Woolley david at ex.djwhome.demon.co.uk.invalid
Tue Dec 30 22:23:19 UTC 2008


Unruh wrote:

> Yes, well... The RS232 standard says that teh signal levels are -12V and
> +12V and that the absolute minimum be -5V and +5V. However, many serial

These are requirements on drivers, not on receivers.

> chip makers have bent those standards and the serial port may or may not

There is no need to bend the requirements in order to produce a receiver 
that is TTL compatible.

> respond to the 0,5 level that the PPS output actually delivers. There is
> absolutely no reason why it should. That signal is completely out of spec.
> If yours does work, it is because your serial port manufacturer severely
> bent the rules. But many (most?) do. But it could well be that the serial
> port is flakey on especially the 0V instead of -5 to -12 V end.
> 

> The parallel port specs state that the signal levels are 0V and 5 V which
> is exactly what the garmin delivers. 

They don't.  On the driver side, they will have a 0 level that is at 
least 200mV.  The 1 level will depend on whether the drivers are open 
collector or totem poll (which I'd have to look up).  They will be based 
on TTL levels, even though modern drivers are CMOS.  The receivers will 
have the standard TTL receiver performance.




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