NMEA 0183 devices are designated as either talkers or listeners (with some devices being both), employing an asynchronous serial interface with the following parameters:

Baud rate:                        4800
Number of data bits:       8 (bit 7 is 0) Stop bits:                         1 (or more) Parity:                              none Handshake:                      none

NMEA 0183 allows a single talker and several listeners on one circuit. The recommended interconnect wiring is a shielded twisted pair, with the shield grounded only at the talker. The standard dos not specify the use of a particular connector. Note: The new 0183-HS standard  (HS = high speed)  introduced in version 3.0 uses a 3-wire interface and a baud rate of 38400. This type of interface is not discussed here.

Its is recommended that the talker output comply with EIA RS-422, a differential system with two signal lines, "A" and "B". Differential drive signals have no reference to ground and are more immune to noise. However, a single-ended line at TTL level is accepted as well. The voltages on the A line correspond to those on the TTL single wire, while the B voltages are inverted (when output A is at +5 V, output B is at
0 V, and vice versa. This is the unipolar RS-422 operation. In bipolar mode ±5 V are used).

In either case, the recommended receive circuit uses an opto-isolator with suitable protection circuitry. The input should be isolated from the receiver's ground. In practice, the single wire, or the RS-422 "A" wire may be directly connected to a computer's RS-232 input. In fact even many of the latest products, like hand-held GPS receivers, do not have a RS-422 differential output, but just a single line with TTL or
5 V CMOS compatible signal level.

8/8/2012 05:29:07 pm

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