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

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Oct 6 05:52:45 UTC 2014

On 2014-10-05 18:22, Rich Wales wrote:
> Is there any way, in the ACTS refclock driver, to specify the speed at which
> the modem's serial port should be opened?
> The source code (ntpd 4.2.7p475) appears to use a hard-coded speed of 19,200
> bits/sec (ntpd/refclock_acts.c, line 138).  I was unable to connect to the
> NIST time service at all -- the service picked up the line, but I heard a lot
> of chaotic noise (suggesting a futile attempt to negotiate a connection) until
> the driver eventually timed out and hung up.
> I can connect without any problems at all, however, if I dial up manually
> (using "cu -s 1200 -l /dev/acts0").
> I changed B19200 to B1200 in the ACTS driver, and the resulting ntpd was able
> to connect and get valid time info from NIST.  But I'd prefer a solution, if
> at all possible, that doesn't require changing the source code.  Any thoughts?

Modem cards would run at whatever interface rate the system set, e.g. 19,200
or 115,200, and use the remote connect info to set the line speed.
The NIST ACTS supported range is 1,200-9,600, and most older modems would
support at least 19,200, so the developer probably picked that speed to
increase support, simplify configuration, perhaps reduce jitter and
increase accuracy.

If you have to pick one speed, use 9,600 for the above reasons; if you want
to support additional options then you could also be compatible with and
dup the NMEA modem flag code and docs for 4,800-115,200.

Speed 9,600 was supported by V.32 thirty years ago, showing just how
old the NMEA and ACTS approaches are (early 1980s and mid 1970s).
My first time sync called ACTS from Canada at that time as it was
cheaper and more accurate than calling NRC in Eastern Canada, and
shorter call duration at 9,600 compared to NRC (still!) at 300.

[Note from Amazon review:
"This is a review of the TrendNET TFM-561U external USB modem,
updated to report the finally-discovered cause and fix for a
slow connection speed on Puppy...Puppy, however, has a problem
- the modem connects to my ISP just fine but was really slow.
PROBE returns a MAXSPEED setting of 460800 and this is the
cause of my slow connection speed on Puppy.
Manually entering a MAXSPEED of 115200 or 57600 cures it.
Think problem is at my ISP's end and not a modem problem.
FYI, either init string works (replace each underscore
character with a space) with MAXSPEED=115200 or 57600.
Can you run it with 19,200 interface speed and 9,600 line speed?
Take care. Thanks, Brian Inglis

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