[time-nuts] GSyncQ GPS Evalution Kits && NTPns

Didier Juges didier at cox.net
Fri Apr 25 08:01:55 EDT 2008

One issue with RS-485 is that the Tx and Rx lines are shared and the driver
chip has to be told which direction it is supposed to work. This is usually
done in the serial driver code using one of the modem control lines. I am
not sure if the NTP code provides that signal. If that signal is not
available, you can use a 555 timer to turn the driver around when it is done
transmitting. Of course, if data flows in only one direction (I do not see
why the PC should talk to the GPS receiver for an NTP application, but there
may be good reasons for that), you can simply tie the driver so that it
works in that direction only.

In a pinch, I have made a RS-485 (one pair Rx/Tx) to RS-232 bi-directional
"converter" with only 3 resistors... I would not pretend it is good
engineering practice, but it works over a few feet (very useful for a quick
verification) and it requires the software in the PC being able to get back
what it sends without throwing its arms up in the air (the human equivalent
of an exception)... I use that "solution" for a project I am working on
right now, and it is nice not to have to carry a bunch of cables/wall warts
and assorted hardware when I carry my project from work to home back and
forth every day.


On that schematic, the right side connector would be the standard serial
connector of a PC. When the RS-485 driver is in the receive mode, it
receives data on the inverting input through the 1k resistor and the other
input is biased to a fixed DC level with the other two resistors. When the
RS-485 driver is talking, it swamps the 1k resistor and drives the Rx pin of
the RS-232 connector directly.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Bruce Griffiths
> Sent: Friday, April 25, 2008 3:12 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GSyncQ GPS Evalution Kits && NTPns
> Pekka
> If you want, you can easily construct your own external RS422 
> to RS232 transceiver.
> Just use a Max 232 to convert the RS232 signals to TTL/CMOS 
> and connect
> RS422 receiver and transmitter chips to that.
> RS 485 transceivers chips can also be used instead of RS422 
> transmitters and receivers, just connect them as drivers or 
> receivers as required.
> If you need a circuit schematic I can send you one.
> Novatel also use Deutsch connectors on some of their enclosures.
> Bruce

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