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.
CAN I USE THE RS232 DTR AND/OR OTHER CONTROL SIGNALS TO POWER
MY GPS?
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