Si-tex SP38, Installation And Operation Manual, Page: 67 / 193

Sitex SP38 Installation and Operation
Document PN 29010101 V1r0
- 67 -
Signal Names
The names of the NMEA data signals, and of the paired wires carrying them (see Table 3 ),
are specified in NMEA 0183: `A´ and `B´. The SPU’s wiring label uses those names, on the
J9– NAV I/O connector (see Figure 30 ). However, sometimes other equipment will use
different names:
Table 3 – 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 (also known as “differential signals”), in paired wires, with the ‘A’ signal on
one wire and the complementary ‘B’ signal on the other.
It is necessary to maintain the correct signal polarity, when connecting one of the SP38
SPU’s RS-422 input or output wire pairs to another NMEA 0183 device:
 The SP38 input port’s `A´ signal pin must be connected to the `A´ output
terminal of the other device; similarly, the SP38’s `B´ input pin must be
connected to the `B´ output terminal of the other device.
 Similarly, the SP38 output port’s `A´ and `B´ signal pins must be connected to
the `A´ and `B´ input terminals of the other device.
Other points to be aware of:
 There is no ground wire for the SP38’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´ and `B´ signal pair to function
properly.
 The SP38’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.
 The CH pin on J6 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 the J6 CH pin).
NMEA Name
J6 Connector
Label
Function
Alternative
Names
`A´ signal
I
N
1A or I
N
2A
O
UT
A
Positive voltage with respect
to `B´ is a logical 0
(aka Active or Space or ON
17
)
+ or NMEA +
SIG or S
IGNAL
POS or +
VE
`B´ signal
I
N
1B or I
N
2B
O
UT
B
Positive voltage with respect
to `A´ is a logical 1
(aka Idle or Mark or OFF)
- or NMEA -
RTN or R
ETURN
NEG or -
VE