Re: [gpsd-users] Communication loss with GR-701W

[Benoît Thébaudeau]

Hi Gary,

Le 07/06/2019 à 05:06, Gary E. Miller a écrit :
> So.  We know:
> 
> The GPS UART1 input buffer is not overrunning.
> 
> The GPS UART1 output buffer is not overrunning.
> 
> The GPS UART1 input is seeing overrun errors.

For MON-RXBUF, the protocol spec says "during the last sysmon period". How is 
this period defined?

If there are Rx overrun errors without Rx buffer overrun errors, it means that 
some characters were received before the firmware of the GPS module had a 
chance to move the previous character from the Rx register to the Rx buffer, 
i.e. the firmware was too slow to handle both Rx and other tasks, which could 
be caused by not waiting for its ACKs.

> Seems to me a lot like a bad serial connection to UART1.

Not really. A UART Rx works at its own baudrate, so whatever is sent to it, 
even at wrong baudrates, the Rx just cannot overrun, unless the Rx processing 
is broken or the firmware processing is too slow compared to the incoming flow. 
As there is no flow control, only ACK monitoring can be used to mitigate this, 
and if it is not sufficient, then there is no solution because it is just an 
issue inside the firmware.

> How is the GPS UART1 connected to your host?

The chip is a UBX-G7020 inside a Navisys GR-701W module, which uses a PL2303 
USB interface. And I get these checksum errors with any host.

Best regards,
Benoît