[time-nuts] RE : Re: FASTRAX GPS

Chris Albertson albertson.chris at gmail.com
Wed Jul 2 20:34:41 EDT 2014

>> Hello,
>> AFAIK, the differential variant of RS-232 is RS-485. I'm not sure about
>> the levels.

You might be thinking about RS-422.  That is the one most like RS232.
It is mostly easy to interconnect 232 and 422.
Using 422 it would be easy to send a serial signal a kilometer over
cat-5 wire and at very high speed.  You can even buy boxes that do
this off the shelf.

But in practical terms what you have is a GPS that is likely sending
serial data using TTL levels and a PC serial port with loose enough
specs that TTL levels just happen to work.  Then you want to connect
them with 100' of unshielded cat-5 wire.   It might work, the specs
say it shouldn't but it many times does or you THINK it works because
the error rate is low enough for your application not to notice.

There are many way to make it work, such as not using rs232 at all and
going with 422 or using a MUCH shorter cable with a shield.

The gpS case really pushes this because it includes a second data
channel in the cable (PPS) and the binary protocol runs 24x7 and will
detect and report every error.  Still it might work or it might not
depending on random things like if there is a large elector motor near

What I did was tried the simplest plan.  It failed so I went with the
next easy plan and it worked.  I was prepared to try a few more but
the cable TV lead wire idea worked fine.  I think this is the way to
do it.  Try "plan A' then B, C and so on.


Chris Albertson
Redondo Beach, California

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