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Electronic Flight Instrumentation for Experimental Aircraft Since 1991

EFIS Systems 101- How It All Works

Required Components

Unless a display unit is to be used as a “dummy” screen for an engine monitor or moving map, there are 3 required components of any GRT Electronic Flight Instrumentation System (EFIS):

  1. Display Unit (also called the EFIS unit)
  2. AHRS unit (Attitude Heading Reference System; pronounced A-hars)
  3. Magnetometer or “remote compass”

The screen, controls, data ports, and main system processors are contained inside the Display Unit. Multiple display units may be used in any installation for redundancy or display of different types of information. Most information displayed on one display unit can also be displayed on all the other units via the inter-display link. Display units do not have to be designated for any one type of information. You can choose to display Primary Flight Display (PFD) instruments, engine instruments, or map data on any screen at any time. Many pilot-selectable options for information display are available at the quick touch of a button.

The AHRS and Magnetometer are mounted away from each other, but are pre-calibrated as a pair here at GRT. They work together to provide inertial attitude and the other flight instrumentation data to the display unit(s). The Horizon EFIS series AHRS unit has an aluminum shell that contains the accelerometers and the air data computer that analyzes the pitot/static information. Note that a traditional pitot/static system is still required with any glass cockpit avionics system; the air lines run to the Air Data Computer ports on the AHRS unit. A “Dual AHRS” unit contains two independent sets of accelerometers and air data computers stacked on top of each other. This way, they are forced to share exactly the same pitch and roll attitude, and can better cross-check each other for proper operation. In the photo above, the single and dual AHRS units are next to each other for comparison. The magnetometer is the black plastic module on the left. The color-coded wiring harness included with the single AHRS is in the foreground. It is used to connect the remotely-mounted magnetometer to the AHRS and the AHRS to the display unit. Dual AHRS units come with two magnetometers and two complete wiring harnesses. GRT AHRS units are wired directly to each display unit; there is no “network hub” common with other EFIS brands. A hub may provide simpler installation, but it also serves as a central failure point when both ADAHRS are wired into it. To be truly redundant, both AHRS units must be independently wired to each display unit.

The Sport EFIS system AHRS arrangement is different. It has the same accelerometers and air data computer, but they are contained in a much smaller package inside the display unit itself. This way, the system is lighter and simpler to install. The Sport display unit case is a few inches deeper than the Horizon display unit for this reason.

With any GRT EFIS system, the magnetometer is a separate unit so that it can be mounted away from all electrical and magnetic interference; typically in a wingtip or tail. In addition to the obvious function of magnetic heading, it is a key player in the inertial attitude calculation of the AHRS. The magnetometer must be mounted parallel to the AHRS (and thus the Sport display unit) in both roll and pitch. This is not as difficult as it sounds when aircraft longerons are utilized as references.  

All of these units are wired together using color-coded tefzel wires supplied with the AHRS. The wiring harnesses are provided with most wires already pinned. Some pins need to be added by the installer; this is done for flexibility, because sometimes the full length of wire provided is not always necessary. The D-sub connectors are added after the wires are run through the necessary bulkheads and ribs in the aircraft; this allows for smaller holes in the structure.

Recommended Components

There are 2 highly recommended components of any GRT EFIS system. These are:

  1. GPS - to enable Moving Map functions
  2. EIS Engine Monitor Package - to provide graphical engine display information to the EFIS

GPS position information is required for functionality of the GRT moving map, which is included as free standard software in all display units. As of May 1, 2013, the GRT Sport S200 (primary) display unit comes with an internal VFR WAAS GPS already installed and wired internally to one of the serial ports. The Horizon system requires an optional, external serial GPS module. A GPS module is not included with Horizon systems because some builders choose to use a different GPS device or IFR certified GPS unit to drive their GRT moving map. GRT builds two external GPS modules, both VFR WAAS and uncertified; one of them is a 4 Hz GPS receiver with RAIM capability. These are convenient and simple to mount remotely anywhere in the aircraft. However, any GPS using the standard NMEA RS-232 serial sentence will work with your GRT EFIS, including most handhelds.

If graphical engine information is to be displayed on the EFIS screen, a GRT Engine Information System (EIS) is also required. The EIS unit has 3 push-button controls and a monochrome LCD screen that displays numerical engine data. The EIS was designed to stand alone as an engine monitor display in aircraft without an EFIS system. It may be mounted on the instrument panel to work alongside an EFIS display unit, or it may be mounted elsewhere in the aircraft and function as a remote sensor module only. Either way, it communicates engine parameters to the EFIS display unit via RS-232 serial link. It cannot be programmed through the EFIS controls, so if it is blind mounted, we recommend putting it in a location that is easily accessible during ground maintenance.

How do I wire it all together?

A key element to designing a glass-panel installation is the communication between different

components of the system. The internal circuits of each unit are wired to use D-sub connectors for easy external wiring of required and optional components. The main connectors are referred to in the installation manuals as Connectors A and B. Each pin of the connector is wired to an internal component of the display unit circuitry. Some pins are hard wired to required elements, such as power and ground. Others are designated as serial ports.

GRT EFIS systems can communicate with a multitude of other electronic devices, including radios, transponders, GPS units, autopilots, transponders, ADS-B and XM weather receivers, and more. Some pilots want very few extra devices, while others want as many as they can fit under the panel. The mission of the aircraft plays a large role in what types of devices are wired to an EFIS system– those flying day VFR local flights might want just a transponder and a radio, while IFR cross-country machines will be outfitted with an autopilot, weather receiver, numerous nav and com radios, and an IFR GPS navigator. Most of these devices use the RS-232 serial language to communicate back and forth with the EFIS system.

Data is transported through the serial ports to make the devices work. A stream of serial data is like a sentence, and data packets are like the words. Data packets are transmitted in a predetermined order and frequency. This frequency is known as the baud rate. A device that

communicates at a baud rate of 9600 delivers 9600 coded data packets per second in a sequence that the receiving device expects. The baud rate of the serial port in the EFIS must be configured to match that of the device; otherwise, the “sentences” of data packets will be scrambled. Note that when two devices share one serial port, they must use the same baud rate.

Serial ports are user-defined, meaning that the aircraft builder can choose which devices use each port. Each serial port consists of two pins– a Transmit (OUT) and a Receive (IN)–that exchange information between the display unit and a connected device such as a GPS, radio or autopilot. Devices that receive and transmit information use one “whole” serial port. Some components only require an IN or OUT. For example, the EIS connects to an IN port because it sends information to the EFIS, but the EFIS does not respond back. The other half of the port, the OUT, may be used for something that only requires data FROM the EFIS, such as a transponder that uses encoder data from the EFIS. The Sport SX (as of May 1, 2013) has 5 serial ports and the Horizon systems have 8 serial ports per display unit. During the display unit setup procedure, you will use the setup guide and equipment supplements to select which device uses each serial port and which baud rate it requires. Don’t worry, this process is simpler than it sounds.

Other data ports include USB, which is used for software updates and simple connection of some ADS-B receivers, Ethernet inter-display link, ARINC 429 for advanced IFR avionics, and most recently, Bluetooth wireless. Bluetooth is a new development that shows much promise for communicating with a multitude of wireless devices. The Horizon system currently uses Bluetooth to communicate with Android tablets and phones through the GRT REMOTE flight planning app. The possibilities are staggering. With GRT’s open-interface philosophy of “playing nice with others,” we are always connecting with new devices that are useful to our customers.

So that is the basic rundown of glass cockpit systems architecture. Keep an eye on our website in the coming months for more tutorials and articles on how to design, install, program and use your own glass cockpit EFIS system.

Sport System Required Components

Display Unit


Pitot/Static System

Internal AHRS

Display Unit



Pitot/Static System

Horizon System with Serial Devices


Serial Port 3

4800 Baud Rate


Serial Port 4

9600 Baud Rate

Display Unit



Pitot/Static System

Horizon System Required Components