[ntp:questions] Adjust serial device speed in ACTS refclock driver?

Rich Wales richw at richw.org
Mon Oct 6 01:33:45 UTC 2014


Replying to Harlan Stenn:

> The docs say it connects at 9600, so there is a discrepancy there...

Agreed.

> I know several folks have been using the current driver without
> problems, so I'm curious about what's going on here.  I also know that
> some modems will auto-baud, but that might not matter.  Some will allow
> the modem connect speed to be different from what is used by the serial
> connection to the modem.

I'm using a TrendNet TFM-561U USB modem -- one of the very few modems I
was able to find that is not a "softmodem" / "winmodem" and will work in
a Linux system without driver software.

I suspect this modem's telco line speed may be tied somehow to the serial
connection speed.  TrendNet didn't supply any documentation for the AT
command set used by this modem, so I tried a "standard" telco line speed
command (ATS37=5) -- this command didn't generate an error, but it had no
discernible effect in ntpd.  Again, when I made the driver open the serial
port at 1200 bps, everything worked.

> The NMEA refclock uses mode bits 4/5/6 to control the baudrate, but I
> don't know if 8 baudrate values will be enough.  Should be OK, but we
> might be using all the values.

I assume the following speeds should be more than sufficient:  300, 1200,
2400, 9600, and 19200; maybe also 38400 and 57600.  I'm not sure anyone
would ever really need to use 300, but why not.  So three mode bits should
be fine -- probably use zero (default if no mode is set) for 19200, and
1-6 for the others.

Unless at least one of the known modem time services supports or requires
a very high baud rate, BTW, I'm unconvinced that supporting anything higher
than 9600 is really necessary for this application.

Rich Wales
richw at richw.org


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