ComNav Commander P2 - Appendix 1 _______________________________________________________________________ 181 Interfacing to a PC; PC COM Ports

ComNav Commander P2 & P2VS Installation & Operation Appendices
Document PN 29010074 V4.1 - 181 -
Appendix 1
Interfacing to a PC
The P Series SPU can be connected to a desktop or laptop Personal Computer, for
Navigation control & monitoring purposes, using the J9 – NAV I/O connector (see Navigation
Data on page 79).
The PC would typically be running a Navigation program (e.g., programs from Fugawi™,
Jeppesen®/Nobeltec®, Rose Point Navigation, or similar), and/or Hyperterminal® or some
other “dumb terminal” program.
For best results, connections to a PC should be done with a suitable RS-422 electrical
interface. The interface can be either on a dedicated add-in board, or via an external
convertor box; typically, the latter convert between RS-422 and either RS-232, or to USB, so
that the signals may be connected to the PC’s standard serial “COM” or USB ports (see the
next section for more details on using these types of PC ports).
Various brands of RS-422 interface boards and convertor boxes are available; contact your
ComNav Dealer for details.
PC COM Ports
It is also sometimes possible to wire the standard serial I/O ports on a PC directly to the
SPU’s J9 – NAV I/O connector.
These PC serial ports – commonly referred to as “COM ports” – always use RS-232 signal
levels.
Connecting a PC’s COM port directly to J9 mixes the RS-232 electrical levels of the COM
port with the RS-422 levels of the SPU’s NMEA 0183 NAV port. In general, doing this is not
recommended – but it does work well in many cases. However, it should be noted that there
is some risk of increased electrical noise in the autopilot–PC system, due to the use of the
COM port’s Signal Ground as the `B´ signal – which may create a partial “ground antenna”
effect in the system wiring.
Note: should you encounter any problems, RS-422 ⇔ RS-232 convertors are
available – or, even better, RS-422 ⇔ USB convertors. Contact your ComNav
Dealer for details.
PC COM ports typically use a DE9
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male (a “pin type”, aka DE9-P) connector; older PCs
(especially desktops) may use a DB25 (aka DB25-P) connector instead.
• Laptop PCs usually have only one COM port.
• Desktop PCs sometimes have 2 (or more) COM ports, although newer models
usually only have one COM port. It is possible to add COM ports to most desktop
PCs, by adding a suitable “COM Port Expander” card.
• Many newer PCs – desktops & especially laptops – do not have a real “COM port”.
Instead, they only have one or more USB ports.
For such PCs, an external convertor (a USB “COM Port”, aka “USB Serial Adapter”)
will be required. These convertors typically have a USB “A size” plug at one end, and
a DE9 male connector at the other end. They come with driver software to allow
application programs to access the USB ports as “virtual COM ports”
Most brands of these convertors should work fine. A number of different ones have
been tried at ComNav, all with success.
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These connectors are often referred to as “DB9”, but that is technically incorrect. The letters “B” & “E” designate the
shell sizes; a B shell is much bigger than an E shell – it can hold up to 25 pins, compared to 9 or 15 pins in an E shell.