SN3308 Primary Navigation Display Support Documentation

The SN3308 Primary Navigation Display/GARMIN GNS430 Interface

Overview

The combination of the Sandel SN3308 and the Garmin GNS430 GPS nav/com represents the state of the art in GA navigation and display. It also represents the state of the art in interconnect -- an area which is bound to cause some confusion in understanding and installing this equipment. It would be worthwhile to read the following information if you have to do something that differs from a plain vanilla installation or just for general interest.

ARINC 429 -- What Is It?

ARINC 429 is a low speed data bus (even when selected to "high" speed) which allows digital messages to be sent from a single output ("source") to one or more inputs ("sinks"). It is uni-directional ("half duplex") which means to talk in two directions you need two ARINC 429's -- one for each way. In that regard it is like RS232, which uses separate TX and RX wires.

Each direction utilizes a two-wire plus ground interface which is relatively robust. The interface is unusual in that it is:

• differential (both wires carry equal but opposite voltages)
• bipolar (the signal on each wire can go both positive and negative in relation to ground)
• trinary (there are three signal states -- plus 5, minus 5, and zero)

These characteristics may be of interest only to design engineers but the signals look very pretty on an oscilloscope!

Unlike serial communications protocols such as RS232 which are byte-oriented (sending packets 8 bits at a time) ARINC 429 is word-oriented which means 32 bits are transmitted in a packet. Each packet reserves 8 bits as a message identifier, known in 429 parlance as a "label". Many messages are self-contained in a single word and hence sometimes a complete "message" may actually be a single "label" -- such as Selected Course Label 100 -- but this is not always the case.

Source / Destination

Often a single 429 output will connect to multiple 429 inputs. A dual SN3308 / dual GNS430 installation works this way by connecting the SN3308's single 429 output to all four Garmin 429 inputs -- two in each GNS 430. Because the SN3308 drives four ARINC inputs, this implies that each message transmission from the SN3308 is received by all four inputs. This is not the case.

The SN3308 mostly needs to send a specific label to a specific input port only, and expects the message to be ignored by all other ports. For instance, the SN3308 might want to set the CDI Select from GPS to VLOC in GNS430 #1, but not change the CDI select in GNS430 receiver #2. How does it do this?

Software accomplishes this by use of the "Source/Destination Index" bits which are reserved bits inside each message word. By setting these bits to 1 or 2, the SN3308 can address Receiver 1 or 2 as long as the installer remembers to set up each Garmin receiver's SDI during installation. This will be a common missed item which will make a #2 receiver inoperative. The symptom: No Selected Course from the HSI or no CDI Select function from the HSI, or even worse the wrong receiver selects!

When things are working properly, the receiver which does not match the SDI label simply ignores the message as if it never came.

2 X 2 = 4

The unique architecture of the Garmin 430 essentially houses a separate GPS and VOR/LOC receiver in each box, each with its own 429 input port, giving us 4 separate 429 inputs total. GNS430 #1 has 429 inputs 1 and 2, and GNS430 #2 has 429 inputs 1 and 2. Standard ARINC 429 SDI does not have this many SDI selections, so the SN3308 wouldn't normally be able to direct its messages individually to each of the four possible targets with a single output.

In order to perform this function extended SDI bits have been defined. Now, the SN3308 has a way to send a message to one of four places -- VOR/LOC1, GPS1, VOR/LOC2, GPS2 -- using a single bus. This is how a single pair of wires can control the four separate receivers.

What does the GPS port send to the SN3308?

• GPS Status
• Flightplan Data
• Nearest Airports
• Nearest Navaids
• Distance and Bearing
• CDI Select Data

What does the VOR/LOC 429 port send back to the SN3308?

• Tuning Data
• VOR Bearing
• LOC Deviation
• GS Deviation
• Flags

Notice that L/R and To/From are not sent from the VLOC receiver to the indicator! This allows the indicator to compute its own deviation display and To/From based on the current pilot selected course -- meaning that two SN3308 displays can have two separate courses and two separate deviation displays, all without transmitting anything back to the GNS430. This demonstrates the digital course resolver function built into the SN3308.

So, does the SN3308 transmit anything back to the GNS430? Well, it doesn't have to, except that the GNS430 is driving the autopilot with its own L/R outputs from its Main Analog Port. In order to ensure that this L/R output matches the pilot's HSI L/R needle, the pilot's SN3308 transmits the selected course digitally to the GNS430, and the digital nav converter in the GNS430 again computes L/R for use by the autopilot. In a single SN3308 / Dual 430 installation the SN3308 transmits selected course only to the currently selected VOR receiver using the SDI bits.

In a dual SN3308 installation only one SN3308 gets to operate the GNS430 Main Port and hence the autopilot -- and this is the SN3308 which has its 429 output wired to the GNS430's. The other (copilot's) indicator is a listener only. It can display any of the four Nav sources but does not control the GNS430 main port or course resolver. In the special case where it is desired that either SN3308 can control the autopilot via Pilot/Copilot selector switch, the basic function of this switch is to change the source of the 429 data from the correct SN3308 in order to get control of the receiver's the main port. Rather unusual.

Note again that in an installation where an auxiliary mechanical resolver/CDI is attached to the GNS 430 Main Port, the L/R needle is driven from the receiver's nav converter which is separate from the SN3308 indicator, giving a truly redundant system. When the SN3308 is driving the receiver with the "Selected Course" message on the ARINC 429, the mechanical resolver is disabled so that the course matches the pilot's HSI. If the SN3308 is driving the other receiver (or is turned off) the mechanical resolver is automatically re-enabled.

A Relay?

In a Dual GNS 430 installation the NAV 429 input data is switched from receiver 1 to 2 by an external relay. If this relay were to fail the SN3308 might show a valid course -- but from the wrong NAV receiver. To ensure that this does not occur, a feedback mechanism is implemented in the SN3308 which requires a ground pin from each selected relay to un-flag the SN3308 display when it energizes. This is referred to as Relay Sense and is part of the SN3308 maintenance setup pages.

Setting Up

Tips

If you select GPS on your SN3308 and you don't immediately have navigation data, check the following:

• Is the receiver turned on?
• Did you configure ALL the ARINC 429 items on EACH receiver as shown above?
• Is LNAV-1 and/or LNAV-2 set up as GNS430 on the SN3308 maintenance page?

When you push the NAV select on the SN3308, it should switch the appropriate GNS430 from GPS to VLOC and vice-versa. If it does not:

• Did you wire the 429 output from the SN3308 to both ARINC 429 ports in each GNS430?
• Did you configure ALL the ARINC 429 items on EACH receiver as shown above? (i.e. Inputs as shown and SDI correct?)

When you push the NAV select on the SN3308, does it switch back to another setting after about two seconds?

• Make sure the GNS430 CDI is switching (see above)
• Make sure the GNS430 GPS port is set to GAMA 429 not ARINC 429. This enables the "extended SDI" bits.

When you select between NAV-1 and NAV-2 on the SN3308, are you seeing the deviation from the correct receiver? If always NAV-1:

• Did you wire a NAV-2 Relay?

If so, check its operation.

If it is Not Operating, the NAV display on the SN3308 should say NAV2. If it is not redlined, check that you assigned NAV-2 on the Relay Sense page to the correct pin number.

If NAV-2 is working correctly, fail the relay intentionally and ensure that the redline appears.

Repeat this process for any other relay that may be installed.

Do you have an external CDI connected to the Main Port on either receiver?

• Check that the SN3308 and the External CDI agree whenever the SN3308 NAV indication is selected to that receiver.
• Ensure that in this condition the External Resolver is not operating.
• Deselect the NAV Source from the SN3308. Ensure that the External Resolver now operates independently of the SN3308.