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. http://www.ko4bb.com/Test_Equipment/RS-485_Converter.jpg 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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] 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 > Internal Virus Database is out-of-date. Checked by AVG. Version: 7.5.519 / Virus Database: 269.22.5/1358 - Release Date: 4/3/2008 6:36 PM _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
