Re: [gpsd-users] Ublox EVK-M8T

[Denny Page]

> On Sep 21, 2017, at 16:03, Denny Page <address@hidden> wrote:
> 
> The core issues is that in the default configuration on the EVK-M8T, the 
> RS-232 signal is inverted on the PPS line. In other words, DCD is held high 
> until TOS, at which time it is pulled down for the pulse width. The default 
> duty cycle is 10%, so this results in the a falling edge at TOS, followed by 
> the rising edge 100ms later. As far as the kernel is concerned, it’s a 900ms 
> pulse. :)
> 
> The M8T has a “polarity” value for the time pulse, which defaults to 
> “rising”, but the effect on the DCD signal out of the EVK appears to be the 
> opposite. It’s unclear whether this is particular to the M8T, or if there is 
> an inverter on the EVK’s RS-232 output. I’ll need a scope to know for sure. :)


Of course, if one actually looks at the provided schematic for the EVK-M8T, one 
will find a TI TRSF3223E RS-232 transceiver, which does in fact invert the 
signal coming out of the M8T. One will also find that pin 1 (DCD) and pin 7 
(RTS) of the DB9 are tied together.

When someone gives you high quality documentation like this, I guess should 
look at all of it. :)

Denny