Dynon rx and tx with vp-x - Page 4

dynonsupport · #31 12-30-2012, 01:50 PM

I have to disagree a bit with the statements made in here about shielding and grounds for RS-232.

You'll notice that on the SkyView harness, there is no specific ground for RS-232 ports, nor are the 3' of wire we supply for each of the 5 serial ports shielded. We didn't do this to cheap out, we did it because we don't believe shielding or grounding is needed in 99.99% of installs.

Shielding is there to either keep noise in a cable (like RF coax) or keep it from getting in a cable (like a microphone cable). SkyView uses slew rate limited serial drivers, which means they do a good job on not creating RF noise. Thus, you are not trying to keep noise from getting out of the cable, and if even if this was the issue, this would do nothing to make communications more reliable, which is the issue here.

RS-232 is a high voltage communications protocol (relative to most; I'm not saying it is going to shock you). It uses higher voltages because this makes the system more noise immune, and it specifically gives up power consumption and speed to do this. In order for outside noise to be an issue to RS-232, it has to be noise on the order of volts, and that would be a crazy amount of RF energy to get into a wire, especially one that is only a few feet long, like it would be between a SkyView and a VP-X. You'd likely have lots of other issues if this much RF energy was bouncing around the plane. RS-232 is also a relatively low impedance load so a lot of the noise gets knocked down by this.

As for grounding, the whole aircraft shares the same ground. SkyView has a nice ground to the battery, and so does the VP-X. Again, in order for ground to be an issue, there would have to be noise on the ground on the order of volts, which would cause huge issues in other places long before it caused serial problems. Adding a little 22 gauge ground wire between SkyView and your other serial device isn't going to make any difference against the whole plane as a gigantic ground. Remember, if your grounds are bad, adding a small wire in parallel doesn't magically fix the problem, it just shunts huge current into that small wire and likely causes it to melt. You need to make sure SkyView and your other devices have really good grounds to the aircraft's ground as a whole and not just between one another. The former automatically takes care of the latter.

Southtarnation has done the test where he hooked the output of SkyView back into the input of SkyView. This worked, and showed data being received into SkyView. This proves a lot: SkyView is outputting data, the cable is solid, and SkyView can receive data as well. Shielding and grounds here are not the issue. I realize that this test does not test for ground, but there is no way the VP-X and the SkyView have a 3V difference on their grounds. On top of this, he's having an issue in a system that doesn't even have a radio or transponder on line, so it's not like there is a lot of RF energy in the plane. Finally, he is getting absolutely zero data back from the VP-X when he's hooked to it. Noise will not cause zero data. It will cause corrupt data, but SkyView will clearly show this as received but corrupt data, not no data at all.

Shielding is needed in a lot of places, such as RF transmission lines, and really sensitive circuits like microphones where millivolts of noise are audible. RS-232 is just plain immune to the small voltages that you might encounter with the short runs you make in a homebuilt plane.

I realize there are some simplifications in the above description, and you can imagine corner cases where shielding would help. But when someone says "my RS-232 device flat out never communicates properly and never seems to transmit a single bye," shielding and grounds are unlikely to be the issue.

--Ian Jordan
Dynon Avionics

Bill.Peyton · #32 12-30-2012, 03:58 PM

While I agree with some of what you said Ian, it is still the convention and good practice to shield all data communication, RF and audio wires, whether to help supress radiated emissions or decrease suscepability to the same. It is the gold standard. I would never wire a panel without. Further, it's called out in most every avionics manual I have ever read. In the long haul, its just another form of protection.

Assuming the transponder is not radiating, then I don't recall that there were any other RF sources, so you are correct, this issue does not appear to be a source of noise on the 232 lines, and they appear to be wired correctly.

At this point, after all of the tests you've run Jesse, I would send the VPX back to have it tested and go from there. Please let us know what you find out
Bill

southtarnation · #33 12-30-2012, 04:48 PM

Marc at Vp offered to take a look at it so ill mail it off asap.

With regards to the shielding; it does seem like that shielding couldnt hurt but since the dynon harness serial lines are not, it would mean that i would have to deconstruct the dynon harness to have shielded the full length or add shielded to the harness in which i would be addid length i dont need and still would have the lenght of unshielded too.

Im curious which you did? Did you add to the efis harness with shielded or did your efis's harness already have shielded or did yoo make your own harness?

Like you guys agree, this is not a grounding or shielding issue and seems to be within the vpx settings or hardware.

Thanks for everyones help, Ill report back what comes of this.

Jesse

Vertical Power · #34 01-14-2013, 12:14 PM

Great news, Jesse! We received your VP-X Sport this morning and immediately hooked it up to our Dynon SkyView on our test bench. Without any changes to the configuration of your VP-X, the box worked flawlessly. The serial communications between your VP-X and our SkyView are working fine. We'll ship your unchanged unit back to you today.

So again, when you get your unit back from us, you should be looking for the source of the problem in your wiring. I recommend you try to simplify the topology as much as possible, and get that working first, before gradually approaching whatever complex topology you require. Specifically, try connecting just a single SkyView screen and your VP-X on your test bench. Use the recommended gauge wires, and double and triple check that you are using the correct pins (pin misidentification is the number one problem I've seen customers have).

We'll continue to be happy to help you track down this problem. Let us know what we can do to help.

Best wishes,

Stephen

southtarnation · #35 01-19-2013, 04:23 PM

I haven't got the unit back yet but I was hoping that maybe I had misread the labeling on the actual connector but after looking with a magnifying glass I think I have the right holes.

The holes on the bottom row correspond to the number the bottom right, correct?

Here is my harness. Starting on the bottom left:
25 - empty
24 - empty
23 - empty
22 - serial/white tx
21 - ground/black to dynon
20 - serial/white Rx
19 - pos sensor
18 - pos sensor
17 - pos sensor

Hope someone sees my mistake...

rvator51 · #36 01-19-2013, 06:21 PM

Hi Jesse,

If this is the DB25 (J1) that connects to the VP-X, could you have pins 20 and 22 swapped? pin 20 is serial TX and pin 22 is serial RX (from VP-X), you show them as:

22 - serial/white tx
21 - ground/black to dynon
20 - serial/white Rx

So they may be backwards.

Pin 20 serial TX on the VPX should go to serial RX on the Skyview. (pins 3,5,7, or 9)
Pin 22 serial RX on the VPX should go to serial TX on the Skyview. (pins 4,5,6, or 10)

Bill.Peyton · #37 01-19-2013, 06:37 PM

Jesse,
I believe you have the pins incorrectly connected. Viewed from the back of the connector as it is pictured in the top photo, pin 1 is top left, pin 13 top right, pin 14 bottom left, pin 25 bottom right. This would indicate that the black wire, which you stated was dynon ground, is in pin 18. You have the connector reversed. If you would like, give me a call I pm you my number.

southtarnation · #38 01-19-2013, 06:50 PM

Quote:

Originally Posted by Bill.Peyton

Jesse,
I believe you have the pins incorrectly connected. Viewed from the back of the connector as it is pictured in the top photo, pin 1 is top left, pin 13 top right, pin 14 bottom left, pin 25 bottom right. This would indicate that the black wire, which you stated was dynon ground, is in pin 18. You have the connector reversed. If you would like, give me a call I pm you my number.

Bill. Thank you. I used a magnifying glass and I thought I could confirm that the pins were numerically ordered from left to right on the top and from right to left on the bottom when viewed from the back. Ill change it and if that's it I will need to check the other harnesses. ...and buy you lunch!

Hope you had a good trip to FL, are you home again now?

southtarnation · #39 01-19-2013, 06:58 PM

Quote:

Originally Posted by rvator51

Hi Jesse,

If this is the DB25 (J1) that connects to the VP-X, could you have pins 20 and 22 swapped? pin 20 is serial TX and pin 22 is serial RX (from VP-X), you show them as:

22 - serial/white tx
21 - ground/black to dynon
20 - serial/white Rx

So they may be backwards.

Pin 20 serial TX on the VPX should go to serial RX on the Skyview. (pins 3,5,7, or 9)
Pin 22 serial RX on the VPX should go to serial TX on the Skyview. (pins 4,5,6, or 10)

Thanks for the input. My labels are the dynon side tx and Rx so we are in agreement it just wast clear from my post wich Rx or tx I was specifying.

Bill.Peyton · #40 01-19-2013, 06:58 PM

Jesse,
I gave you a female connector pin out. I was incorrect, in that you have a male connector.

The pins for a male connector are as follows as viewed from rear with the wide side up. Pin 1 top right, pin 13 top left, Pin 14 bottom right, pin 25 bottom left.

In this case your connector is wire correctly, assuming that you have the RX and TX lines on the correct pins.

Sorry about my confusion!

Bill