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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Apr 25 08:16:46 EDT 2008

Didier Juges wrote:
> 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

Look at the new circuit I've just added to the miscellaneous section.
I've just used the RS485 drivers not the RS485 standard.

The GPS receiver actually uses dedicated RS422 transmit, PPS, and 
receive pairs.
I've merely used RS485 transceiver chips for convenience and their 
higher current drive capability.
The RS485 transceiver signal levels are RS422 compatible.


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