ComNav G2 - Page 40

ComNav G2 & G2B Installation & Operation
Document PN 29010093 V1.5 - 38 - 16 December 2016
The PC would typically be running a Navigation program (e.g., programs from Fugawi™,
Jeppesen®/Nobeltec®, Rose Point Navigation, or similar), replacing the Navigation
control functions of (or as a backup to) the Chartplotter.
In this connection scheme, the PC will need to be hooked to the G2 via the latter’s
RS-232 connections – but it will also usually be necessary to connect the PC to the
autopilot and/or other NMEA 0183 equipment, at the NMEA Standard’s RS-422 signal
levels.
⇒ Since very few PCs have built-in RS-422 ports, an RS-232 ⇔ RS-422
Convertor will usually also be needed (as shown in Figure 17).
Alternatively, if your PC has USB ports, a USB-to-422 adapter could be used
for the PC-to-autopilot/etc. connection.
Contact your ComNav Dealer for ordering information.
Note: it is sometimes possible to mix the RS-232 electrical levels of a PC’s COM port
with the NMEA 0183 Standard’s RS-422 levels – driving into an RS-422 input from
the PC’s RS-232 output, and/or vice-versa.
For an example, see the section describing NMEA connections to the Commander
P2 SPU’s NAV ports, in the P2’s Installation & Operation Manual. This method
usually works fine … on the P2 (& also on the Admiral P3), at any rate.
But it depends largely on the electrical drive capabilities of the PC’s COM port
circuits. As well, in a technical sense, it violates the rules of the NMEA 0183
Standard, since the voltage levels, signal rise/fall times and circuit impedances of
RS-232 and RS-422 are different. It may also cause electrical noise problems –
RS-232 is “single-ended” wiring, whereas RS-422 is “differential” (i.e., twisted-pair
wires) and thus withstands RFI & EMI electrical noise better.
For those reasons, ComNav strongly recommends against mixing RS-232 &
RS-422!.
However, if you wish to try it with a P2 or P3 SPU:
• Wire the G2’s Port A RS-232 Tx signal to the PC COM port’s Rx signal, and
wire the RS-232 Signal Ground, both as shown in Figure 17.
• Wire the PC COM port’s RS-232 Tx signal to the IN-2A pin of the P2/P3
SPU’s J9, and the PC’s RS-232 Signal Ground to the SPU’s IN-2B pin.
• If the PC has only one COM port, but you need to run both “Navigation” &
“G2 Control” programs on the PC (see below), you can also connect the PC’s
Tx signal & Ground to the G2’s Port A RS-232 Rx & Signal Ground wires, in
parallel to connecting them to a P2/P3.
Caution! You can NOT do the same for the Rx signal – i.e.,
connect both the G2’s Tx signal and the P2/P3’s NAV OUT
signal to the PC’s Rx pin. Doing so will damage either the
G2 or the P2/P3 … or both.
If you encounter any problems with this wiring scheme, use
of an RS-232 ⇔ RS-422 Convertor will then be required.