Re: RS232 convertor - Why?

On Sun, 03 Dec 2006 10:19:24 +0800, NMEA0183 <me@xxxxxxxxxxx> wrote:


NMEA I/O from a GPS receiver is TTL swing but opposite polarity to true RS232.
So apart from the level shifting - which in MANY cases is not required - the
inversion in a MAX 232 (et al) is the key ingredient in most of these circuits.

NMEA-0183 is TTL levels, but with RS-232 polarity. Almost all PC
serial ports will accept the 5 volt NMEA-0183 output from a GPS
receiver with no problems.

To convert NMEA-0183 signals to true RS-232 levels, you need an
inverter and a MAX 232 or other RS-232 line driver, as the line
drivers invert the signals, as well as changing levels.


Regardless of actual signal levels, I'd be betting that the camera has true
RS232 polarity at its serial port and can direct connect to a PC's com port.
The odd-man-out in these schemes is the GPS receiver. The NMEA signalling
system was designed to directly interface (and multi-drop) to other marine
electronic devices such as autopilots, NOT personal computers. Some later GPS
receivers have USB output to satisfy the market for direct-PC-connection.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
.

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