pySerial Windows write problem

[Peter Hansen]

Bob Greschke wrote:
> "Peter Hansen" <peter at engcorp.com> wrote in message >
>>I'd call it very unusual 
>>(in my experience) to have a program open and close a serial port 
>>repeatedly.  
> 
> One of the serial ports (there are actually two) is used to read some NMEA 
> sentences from a GPS.  It is only rarely read.  If it is also opened when 
> the program starts and kept open would you just dump the buffer and then 
> read to get the most current info?  What happens when the buffer fills up? 
> The "main" port is just commands sent, responses received kind of traffic.

Generally, yes, you just dump data received up to the point you are 
about to send a new request (if this is a request/response type of 
thing).  PySerial has a .flush() method of some kind I believe, or you 
can just loop as long as inWaiting() isn't False.

If you aren't using hardware or software handshaking, then the buffer 
filling up is a non-issue for you or the sender.  If you're using 
handshaking, then you do have to keep the buffer from filling.  Pretty 
much all my interesting code runs the PySerial stuff in a separate 
thread, reading whenever data is available, and (this is a 
simplification) posting it to a Queue which the main thread can read 
from as required.  In that scenario, you could just have a flag that 
causes the receive thread to stop posting stuff to the Queue where you 
currently have close the port to prevent data being seen.

I don't recall: is NMEA 0183 asynchronous?  Messages can be sent by the 
GPS even when you didn't ask for one explicitly?  If so, then it's 
possible there are problems even with your current approach: what if you 
open the port halfway through a packet, and receive only the last few 
bytes?  If it's synchronous (data sent only in response to your 
requests), then none of this is an issue since there will be no traffic 
unless you ask for it, so the serial port will be idle between uses.

-Peter