In article ,
Anonymous wrote:

Unfortunately, whereas "RS-232 and NMEA" are very much a standard,

Hi, Joel,

Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard. For example, the waveform coming
out of the GPS receiver was +/- 5 V, whereas I have seen the waveform out of
the RS-232 on the computer at +/- 12 V. Is the voltage level arbitrary?

The RS-232 specification formally requires that the voltage swing both
positive and negative, with respect to ground, by at least 5 volts.
Swings for a loaded circuit can go up to +/- 15 volts, and the swing
must not exceed +/- 25 for an unloaded circuit.

RS-232 receivers are required to be sensitive enough to detect
reliably at +/- 3 volt levels. Voltages between +3 and -3 are
somewhat of a "no-man's land" and as far as I know the RS-232 standard
is silent on how they should be interpreted.

The slightly newer RS-423 spec also requires positive and negative
swings, but they only need to be +/- 3.6 volts at a minimum, +/- 6
volts at a maximum, and the receivers are required to have +/- 200
millivolt sensitivity. This standard allows for simpler operation
from a 5-volt-only circuit (a simpler charge pump can generate a -5
supply), and the increased receiver sensitivity allows the system to
work with longer lines (up to 4000 feet, where RS-232 is formally
limited to 50 feet).

A lot of devices these days don't produce properly-conforming RS-232
*or* RS-423 signal levels on their serial ports. It's fairly common
to see devices which use TTL/CMOS voltage levels - swinging up no
higher than 5 volts, and down to ground.

To cope with such nonstandard signals, it has become fairly common to
use serial-port receivers which are deliberately asymmetric... they
have a single detection threshold, somewhere in the 1-volt range I
think. A receiver of this sort will work properly (with a slight
reduction in noise margin) with a real RS-232 or RS-423 transmitter
which swings to +5/-5 or above, and will also work with a "pseudo-RS-
423" transmitter which swings +5/0.

--
Dave Platt AE6EO
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