Oh boy, here comes a rabbit hole! I've been down this rabbit hole myself.
GRT does some interesting things with magnetometer calibrations and the inter-display link (hereafter known as the IDL).
Firstly, one has to understand the IDL is a daisy chain. Output of EFIS 1 goes to input of EFIS 2. Output of EFIS 2 goes to input of EFIS 3. Output of EFIS 3 goes to input of EFIS 1.
If one is using a single magnetometer (the newer digital magnetometer) its single RS232 serial output (Pin 9) can be split to as many displays as you have.
EDIT: this is RAW magnetometer data which MUST go to each display. This RAW data is then massaged inside each display to produce the displayed heading. It's the MAGCAL (magnetometer calibration) data which allows the RAW magnetometer data to be corrected for errors in the magnetometer and for local magnetic field disturbances resulting from the magnetometer installation.
With IDL turned on between displays there is a CRITICAL setting in IDL setup which must be considered. This setting is "SHARE MAG CAL ON INTERDISPLAY LINK". If set to YES then the magnetometer calibration information is shared from the IDL MASTER to the other EFIS devices. What this accomplishes is that it causes multiple displays to use the same calibration data for the same magnetometer. This happens as a check at power-on. If MAGCAL data is coming across the IDL then the slave displays will use that MAGCAL data and the old MAGCAL data which might be stored in their memory is put back on the shelf and not used.
Now here is where it gets tricky. When doing the magnetometer calibration in an aircraft equipped with ONE magnetometer and MULTIPLE displays the following process needs to be followed. Firstly, set the IDL parameters properly to show which devices are slaves and which is the master. Turn ON "SHARE MAGCAL OVER INTERDISPLAY LINK" on all displays. Power all displays off.
Power on only the master display. Perform the magnetometer calibration by going around in a circle per the installation manual. Once this is done make sure you SAVE the calibration data. Now turn off the master display. What has happened here is, within the IDL master, a lookup table of data values has been created; this lookup table is a lot like a compass correction card in that it tells the display how to correct the raw magnetometer data in order to produce accurate displayed heading information. If we have multiple displays using the same magnetometer, each of those displays is going to be fed raw magnetometer data which contains the same errors. Each display then needs to apply the same "compass correction card" information to correct that raw data so each display shows accurate heading information.
Turn all displays on. What's happening now in software is the IDL master is spewing out a bunch of data for the other displays... including MAGCAL data. The other displays know they are slaves and they have been waiting for the MAGCAL data over the IDL. Within a very short time after power-up the slave displays will have received MAGCAL over the IDL and will have applied that "compass correction card" information to the raw magnetometer data they are all receiving from the shared RS232 signal coming from magnetometer Pin 9. The heading indicated on each display should match that of the master, plus or minus a degree or two. (I know, we think they should match exactly but they don't, thanks to some very fancy math that's being done in the background - but honestly, when was the last time you could fly your aircraft and hold heading within one degree?)
If you don't follow this process you will end up with contention in the slave displays as they won't be able to properly load the MAGCAL data from the IDL master. This will result in wild heading displays.
This gets even more complicated if one has a more complex installation like mine where I have one magnetometer shared between two displays and a third display with its own magnetometer - this condition is one which requires a different process, including disabling sharing of MAGCAL on the IDL.
Thanks Mark, This also explains why we have to designate MASTER and SLAVE (never really made sense to me as the IDL is a two way street).
On last question - Since both Displays in my setup are connected to the serial stream of the magnetometer - is there any reason I couldn't calibrate each display individually (ater turning off the share cal data)?
..because a single display provides power to the magnetometer.
For the sake of my education: would appropriate diodes and shorting the grounds together make this a simple possibility understanding there remains a single point of failure?
Thanks,
Mike
So just to go further down this rabbit hole....
I asked GRT about the Master/Slave designation and this is what they said:
"The primary/slave thing is mainly about which one starts communicating first and which one settles disagreements. As long as you have one primary and the other as something else it's fine."
And the Orientation of the magnetometer needs to be set in each display [I have a Sport and a Mini]:
From GRT:
"The Sport and Mini both have to be told about the magnetometer orientation using SET MENU, AHRS Maintenance, Magnetometer Orientation, Magnetometer Roll/Pitch. The orientation data is computed and stored in each AHRS, not in the magnetometer, and not shared. If the magnetometer isn't mounted flat on its bottom (default orientation), or if it was previously configured with a magnetometer in a different orientation, then the headings will be wildly incorrect until you do this."
In my case, I fixed the heading issue by following Mark's advice. First I made sure the orientation was correct in both displays, then ran fine calibration on the primary display with the second display off, then powered down the primary; then started both displays. THANKS MARK!
For what it's worth, the digital magnetometer is not hugely expensive and it weighs nothing, so if one really wants redundancy (I did), one installs a second magnetometer, thereby removing a single point of failure from the system.
So just to go further down this rabbit hole....
I asked GRT about the Master/Slave designation and this is what they said:
"The primary/slave thing is mainly about which one starts communicating first and which one settles disagreements. As long as you have one primary and the other as something else it's fine."
And the Orientation of the magnetometer needs to be set in each display [I have a Sport and a Mini]:
From GRT:
"The Sport and Mini both have to be told about the magnetometer orientation using SET MENU, AHRS Maintenance, Magnetometer Orientation, Magnetometer Roll/Pitch. The orientation data is computed and stored in each AHRS, not in the magnetometer, and not shared. If the magnetometer isn't mounted flat on its bottom (default orientation), or if it was previously configured with a magnetometer in a different orientation, then the headings will be wildly incorrect until you do this."
In my case, I fixed the heading issue by following Mark's advice. First I made sure the orientation was correct in both displays, then ran fine calibration on the primary display with the second display off, then powered down the primary; then started both displays. THANKS MARK!
Oh boy, here comes a rabbit hole! I've been down this rabbit hole myself.
GRT does some interesting things with magnetometer calibrations and the inter-display link (hereafter known as the IDL).
Firstly, one has to understand the IDL is a daisy chain. Output of EFIS 1 goes to input of EFIS 2. Output of EFIS 2 goes to input of EFIS 3. Output of EFIS 3 goes to input of EFIS 1.
If one is using a single magnetometer (the newer digital magnetometer) its single RS232 serial output (Pin 9) can be split to as many displays as you have.
EDIT: this is RAW magnetometer data which MUST go to each display. This RAW data is then massaged inside each display to produce the displayed heading. It's the MAGCAL (magnetometer calibration) data which allows the RAW magnetometer data to be corrected for errors in the magnetometer and for local magnetic field disturbances resulting from the magnetometer installation.
With IDL turned on between displays there is a CRITICAL setting in IDL setup which must be considered. This setting is "SHARE MAG CAL ON INTERDISPLAY LINK". If set to YES then the magnetometer calibration information is shared from the IDL MASTER to the other EFIS devices. What this accomplishes is that it causes multiple displays to use the same calibration data for the same magnetometer. This happens as a check at power-on. If MAGCAL data is coming across the IDL then the slave displays will use that MAGCAL data and the old MAGCAL data which might be stored in their memory is put back on the shelf and not used.
Now here is where it gets tricky. When doing the magnetometer calibration in an aircraft equipped with ONE magnetometer and MULTIPLE displays the following process needs to be followed. Firstly, set the IDL parameters properly to show which devices are slaves and which is the master. Turn ON "SHARE MAGCAL OVER INTERDISPLAY LINK" on all displays. Power all displays off.
Power on only the master display. Perform the magnetometer calibration by going around in a circle per the installation manual. Once this is done make sure you SAVE the calibration data. Now turn off the master display. What has happened here is, within the IDL master, a lookup table of data values has been created; this lookup table is a lot like a compass correction card in that it tells the display how to correct the raw magnetometer data in order to produce accurate displayed heading information. If we have multiple displays using the same magnetometer, each of those displays is going to be fed raw magnetometer data which contains the same errors. Each display then needs to apply the same "compass correction card" information to correct that raw data so each display shows accurate heading information.
Turn all displays on. What's happening now in software is the IDL master is spewing out a bunch of data for the other displays... including MAGCAL data. The other displays know they are slaves and they have been waiting for the MAGCAL data over the IDL. Within a very short time after power-up the slave displays will have received MAGCAL over the IDL and will have applied that "compass correction card" information to the raw magnetometer data they are all receiving from the shared RS232 signal coming from magnetometer Pin 9. The heading indicated on each display should match that of the master, plus or minus a degree or two. (I know, we think they should match exactly but they don't, thanks to some very fancy math that's being done in the background - but honestly, when was the last time you could fly your aircraft and hold heading within one degree?)
If you don't follow this process you will end up with contention in the slave displays as they won't be able to properly load the MAGCAL data from the IDL master. This will result in wild heading displays.
This gets even more complicated if one has a more complex installation like mine where I have one magnetometer shared between two displays and a third display with its own magnetometer - this condition is one which requires a different process, including disabling sharing of MAGCAL on the IDL.
Lars - I think we've covered the high points here when it comes to installing the second magnetometer.
0) configure EFIS serial ports correctly to talk to the magnetometer
1) set magnetometer orientation
2) turn off IDL SHARE MAGCAL
3) perform MAGCAL fine calibration on each EFIS system independently, while the other is powered off
4) don't forget to SAVE that MAGCAL data at the end of the calibration run!
Does anyone know if this procedure is still required?
I've just upgraded from Sport HS EFIS to Sport EX EFIS.
I have the digital magnetometer connected to both EFIS. I did the magnetometer calibration on the primary EFIS. They both seem to operate as expected.
But I was unable to find the IDL SHARE setting so have not done the second calibration.
Thanks
Jim Butcher
I have a SportEX and MiniAP sharing one magnetometer...
There was a massive change log associated with the July 2022 update. The latest software tries to update the magnetometer orientation automatically when you calibrate it. The AHRS measures relative MAG orientation for about 10 seconds and stores it. A quick check is to check the AHRS MAINTENANCE - MAGNETOMETER CAL DATA SYNC STATUS to see if the displays recognize that they are sharing the same magnetometer and if the magnetometer correction data is synchronized. When you do the MAG Calibration on one display, that EFIS tries to automatically sync the correction data if both screens are on at the same time. If you haven't updated to the July 2022 release, I suggest you do so. I had a discrepancy with my inflight wind calculations and reached out to GRT (a lot of what I typed came from Jeff DeFouw at GRT). After updating to the latest SW release and redoing my fine MAG CAL, both screens are accurately reporting heading and winds aloft. Hope this helps.
Dave
-8 flying
Woodbridge VA
I haven't yet updated the software in my HX/SportEX/Mini-X. I have a question into Jeff DeFouw to clarify magnetometer calibration procedures in my particular mix of equipment. I'll report back here when I have a more complete understanding of how this new software works.