RS-232C Signal Levels versus the NMEA levels used by GPS

RS-232C (the "C" spec revision) RECEIVER devices are designed to operate from +3vdc to +25vdc for the "high" logic level and about +1V to -25Vdc for the "low" logic level.  This spec was changed from the RS-232B spec MANY years ago to permit inter operation of a  serial RS-232 port with  a TTL logic signal.  However,  the normal RS-232C noise margins are not maintained when TTL signals are interconnected.   "Almost" all computer manufacturers design their RS-232 serial port to operate with TTL logic signals and all of the consumer model GPS receivers we have examined use a "TTL or CMOS type" logic signal for the serial output signal.  [Note:  The transmitters of RS-232C devices normally output from +6 to +12volts for the "high"  level (space on data channel) and from -6vdc to -12vdc for the "low" level (mark on data channel).  GPS receivers are designed to accept these levels on the serial input and operate properly without damage.]

By Comparison,  NMEA signal output voltages run from about +5vdc to 0vdc.  Thus,  if you feed this signal into a RS-232C input,  then when the NMEA signal is at +5vdc,  you will get one logic level and when it is 0vdc,  you will get the other out of your RS-232C receiver.  The noise margins are not up to RS-232C standards,  but with reasonably  short cables (lets say 10 feet and less) in "normal" environments,  all will work well.   If the equipment is other than handheld GPS equipment OR IF THE EQUIPMENT IS DESIGNED FOR PRIMARY NAVIGATION USE WHERE SAFETY OF LIFE IS A CONCERN, then the manufacturers of the equipment should be consulted to insure that the two equipments will properly interconnect.

In any case,  bonding of the common ground of all permanently mounted equipment is required to insure that all equipment shares a common ground potential.

I have been told that there is an RS-232D specification out now,  but I am not aware of differences between this and the older RS-232C specification.


No.  The RS232 outputs are current limited at about 10ma and the output voltage will drop substantially even at that output current level.  You may be able to use maybe 5ma max to power a device,  but there is no way to get the 100ma or more needed to power a typical GPS receiver.

Joe Mehaffey

updated 3/25/00