ComNav Commander P2 - Figure 63 - Typical RS-422 Signals; Typical RS-422 Signals

ComNav Commander P2 & P2VS Installation & Operation Installation
Document PN 29010074 V4.1 - 81 -
Typical RS-422 Signals
The only way to troubleshoot problems with the RS-422 NMEA signals to & from the
Commander P2 is with an oscilloscope; that’s because a multimeter is essentially useless for
differential signals. The figure below is an example of a typical NMEA 0183 RS-422
transmission, captured on a dual-trace oscilloscope.
The data stream is from a P2 SPU’s NAV - OUT port. Channel 1 is the RS-422 `A´ signal,
Channel 2 is the `B´ signal, and the Math trace is the differential voltage between `A´ and `B´;
the ‘scope probes were manually set at 10x, so the voltages of the signals are 10 times larger
than measured below. The Ground Reference for the ‘scope’s inputs was the GD pin of the
J9 – NAV I/O connector. The Baud Rate was 4800 (or ~208.3 msec per bit).
The data being sent is the first two characters at the start of the NMEA sentence: $A (the full
sentence might have been $APHDM,346.5,M*37 or something similar – see page 188).
In hexadecimal, $A is: 0x24, 0x41; in binary: 0b00100100, 0b01000001.
Following the rules of NMEA 0183, that binary data was transmitted starting with the least
significant bit of the first character, in 8-bit groups: 00100100, then 10000010 (as shown in
the blue overlay in the figure below). The transmitter circuit demarked each group (“byte”) by
a leading `0´ Start bit and a trailing `1´ Stop bit. Finally, the train of bits was sent to the
RS-422 differential driver output circuit, which put inverted bits on the `A’ signal line, and
non-inverted bits on the `B´ line.
Before transmission from the RS-422 driver started, the signals were in the Idle state: `A´ was
at a low voltage with respect to `B´ (the same state as a `1´ bit). After all the bits of the full
sentence had been transmitted, the signals would have returned to the Idle state.
`A´ Signal
`B´ Signal
Differential
Signal
(`B´ - `A´)
StopStart
0 0 1 0 0 1 0 0
StopStart
1 0 0 0 0 0 1 0
Start
0
Idle
Figure 63 – Typical RS-422 Signals