ComNav Commander P2 - RS-422 Electrical Interface; Signal Names; Table 4 - NMEA Signal Translation Guide

ComNav Commander P2 & P2VS Installation & Operation Installation
Document PN 29010074 V4.1 - 80 -
Signal Names
The names of the NMEA data signals, and of the paired wires carrying them (see below), are
specified in NMEA 0183: `A´ & `B´.
The SPU’s wiring label uses those names, on the J9 – NAV I/O connector (see Figure 35).
However, sometimes other equipment will use different names:
NMEA Name
J9 Connector
Label
Function
Alternative
Names
`A´ signal
IN 1A or IN 2A
OUT A
Positive voltage with respect
to `B´ is a logical 0
(aka Active or Space or ON
17
)
+
or
NMEA
+
SIG or SIGNAL
POS or +VE
`B´ signal
IN 1B or IN 2B
OUT B
Positive voltage with respect
to `A´ is a logical 1
(aka Idle or Mark or OFF)
-
or
NMEA
-
RTN or RETURN
NEG or -VE
Table 4 – NMEA Signal Translation Guide
RS-422 Electrical Interface
The RS-422 electrical interface specified in the NMEA 0183 Standard uses a pair of balanced
signals (aka “differential signals”), in paired wires, with the ‘A’ signal on one wire & the
complementary ‘B’ signal on the other.
It is necessary to maintain the correct signal polarity, when connecting one of the
Commander P2 SPU’s RS-422 input or output wire pairs to another NMEA 0183 device:
• The P2 input port’s `A´ signal pin must be connected to the `A´ output terminal of the
other device; similarly, the P2’s `B´ input pin must be connected to the `B´ output
terminal of the other device.
• Similarly, the P2 output port’s `A´ & `B´ signal pins must be connected to the `A´ &
`B´ input terminals of the other device.
Other points to be aware of:
• There is no Ground wire for the P2’s NAV1 - IN ports. Both ports are NMEA Listeners
(as defined in the Standard), and so are optically isolated from the rest of the SPU
circuitry; they need only the `A´ & `B´ signal pair to function properly.
• The P2’s NAV - OUT port (an NMEA Talker) is not optically isolated from the rest of
the SPU circuitry. However, since the other device will have an optically-isolated
input (if it is a fully-compliant NMEA Listener), no Ground wire for the NAV - OUT port
is necessary
18
.
• The CH pin on J9 is connected directly to the SPU chassis, and is also AC-connected
to the SPU’s Signal Ground, via a 100 nF capacitor. If the cable carrying the NAV
signal wires from/to the other NMEA device is shielded, the shield should be wired to
the CH pin.
 The cable’s shield must not be connected at the other end, or anywhere else
but to J9’s CH pin!
17
An NMEA 0183 serial data stream is inverted by the transmitter, and inverted again by the receiver, and so a logical ‘1’
in the data appears as a logical ‘0’ on the `A´/`B´ wire pair.
18
The “ground reference” voltage for the output signals is available on the GD pin of J9 – but note that this is not the
SPU’s signal Ground, rather it is a synthesized reference voltage, and so it must not be connected to any other Ground.