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RS232 Signal Ground

TASEsq

Well Known Member
Patron
I'm planning wiring for a G3X system and looking at backup rs-232 data paths - confused by how the RS232 works (if it needs a dedicated ground).

In some diagrams of the G3X manual, such as on 27-7 for the path between the GSA28 and the GMC307 or the GMC307 to the GDU, it shows the rs-232 as a 2 core shielded wire (shield grounded to the connector at both ends) and does not use any "SIGNAL GROUND" pins, or 3rd wire for ground.

In other areas, like the GDU to the GEA24 such as on 27-11, it shows the use of a 3 core shielded wire (utilising a "signal ground" pin at each end), with the shield ground to the connector at both ends as well.

Now on the GEA24, J241 connector there is no signal ground pin!

Do i use a 2 core wire for this? or do i use a 3 core wire and connect the "signal ground" pin on the GDU to either "ground" pin on the GEA 24? (power ground?)

I guess a basic question - what is a "signal ground" pin? Is this just a pin which is grounded to the unit internally? (aka the same as the power ground etc).
 

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I guess a basic question - what is a "signal ground" pin? Is this just a pin which is grounded to the unit internally? (aka the same as the power ground etc).

There is no universal answer. Yes, some manufacturers may supply a ‘signal ground’ connection which is just a convenient place to attach a wire to the general ground. Others, like some audio panels, may electrically isolate signal grounds from the general ground. ARINC circuits isolate the return line so well they don’t even call it ground - just A and B lines. Some RS232 circuits may attempt to have dedicated return lines, but most are corrected to the common ground.
 
Do i use a 2 core wire for this? or do i use a 3 core wire and connect the "signal ground" pin on the GDU to either "ground" pin on the GEA 24? (power ground?)

I guess a basic question - what is a "signal ground" pin? Is this just a pin which is grounded to the unit internally? (aka the same as the power ground etc).

Trent,

Either method (2 conductor or 3 conductor) will work fine. In the case of the GEA 24 connector P241, power and signal ground are the same.

If you use 3 conductor shielded cable, you can connect the RS-232 signal ground to pins 6 or 9 on GEA 24 connector P241. You may need to terminate 2 grounds on one of these pins if you are supplying dual power sources to the GEA 24. I like the "window strip" method for doing this.

Steve
 
RS-232 is a "single ended" signalling scheme and requires a ground reference to work properly (or at all)

This reference is generally achieved in one or more ways:

A) Direct ground tie, via 3 wire twisted pair, with or without a shield. This is the most robust and it makes no assumptions about the power supplies or chassis grounds between the two boxes -- the can be running on two separate batteries with no other electrical ties than the RS-232 lines, and communication will work great.

B) Ground tie via twisted pair shield. Typically, the metal connector shells are grounded to the chassis of their respective boxes and to the signal ground within. Terminating the shield at both ends to the connector shells will tie the grounds together without an explicit wire. The Garmin all-metal backshells provided with the various HW kits make this ground tie very easy to make.

C) Ground tie via the power supply. For our planes, the power returns from the various boxes are most often tied back to one point (battery/forest of tabs), tying the various signal grounds together. This is the least robust method since the common ground point can be quite far from the boxes (think 10's of feet), which provides ample opportunity for EMI and resistive voltage drops in the return lines to inject noise onto the reference.

Attached is cartoon schematic illustrating the connetions outlined above. On my plane, I did A when I could (twisted pair with shield), B when I couldn't, and I got C for free via the system's common power return point. Though C alone may work "most of the time" I wouldn't trust C alone to provide reliable communications, especially at higher baud rates.
 

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Thank you - that’s all very helpful.

I understand why Garmin drew their diagrams like that now.

So to get myself clear:
The GEA24 does not have a signal ground pin, only 2 power grounds.
The GDU460 does have a signal ground for each serial port.

In this case, it would be fine to use a 2 core wire and ground the shield at each end to the connector. This will work because I am running all the grounds back to a common source.

The only query I had on this was the use of the “keep alive” function of the gad27. Most units will have a primary power source and ground, and a keep alive power source. I assume this is fine as the gad27 would also return to the same shared grounds.
 
In this case, it would be fine to use a 2 core wire and ground the shield at each end to the connector. This will work because I am running all the grounds back to a common source.

It works because you've tied both Garmin box signal grounds together via the shield ties at the connectors (my option "B") -- Garmin's connector shells are tied to ground. While the ground tie back to the common source is there as you note, it's not necessary with your proposed connection.
 
It works because you've tied both Garmin box signal grounds together via the shield ties at the connectors (my option "B") -- Garmin's connector shells are tied to ground. While the ground tie back to the common source is there as you note, it's not necessary with your proposed connection.

Ok - got it thanks.

So at the display unit, which has signal ground pins, if I wanted a cleaner install I could ground the shield of the 2 core cable to a signal ground pin - basically doing the same thing as grounding it at the connector backshell. At the other end, where I don’t have a dedicated signal ground pin I can ground the shield to the backshell.

Or if it were some other 3rd party non-Garmin device, use a signal ground pin to the shield at the display end, and ground the shield to a power ground for the device at the other end. (Or, use a dedicated 3 core wire)
 
Ok - got it thanks.

So at the display unit, which has signal ground pins, if I wanted a cleaner install I could ground the shield of the 2 core cable to a signal ground pin - basically doing the same thing as grounding it at the connector backshell. At the other end, where I don’t have a dedicated signal ground pin I can ground the shield to the backshell.

Even though it's an extra wire, I like the two signals plus ground connected directly to the connector pins, with the braid separately tied to the backshell. It has the best signal properties, and keeps the braid as a shield from external noise sources -- it doesn't have to do double duty as shield+ground. In this example, you can terminate the braid at just one end; don't need to do it at both ends if that's not convenient.
 
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Very Helpful

Guys,

Helpful and timely posts

This is exactly where I am in creating the wiring schematic. The installation manual doesn't explicitly state the reason for multiple signal grounds on Garmin units or the internal physical connection, or lack thereof, between the various signal grounds and the "GROUND" ground. Do they all share a common ground, are they each in some mysterious way different, etc.

Thanks for the info!

Bob
 
RS-232 Ground?

There is no universal answer. Yes, some manufacturers may supply a ‘signal ground’ connection which is just a convenient place to attach a wire to the general ground. Others, like some audio panels, may electrically isolate signal grounds from the general ground. ARINC circuits isolate the return line so well they don’t even call it ground - just A and B lines. Some RS232 circuits may attempt to have dedicated return lines, but most are corrected to the common ground.

In General, RS-232, is an intercommunication means, by which, data is "transferred" in series method - bit after bit.
As an electrical signal, it should have "reference" to which the signal is "referred to".
In order to accomplish this, there must be two electrical lines: GND and SIGNAL. The SIGNAL line is carrying +/- 4V up to +/-12V referring to the GND line.
Sometimes we want the data to be transferred in both directions at the same time. If this is the case, we need two signal lines: one to transmit from equipment "A" to equipment "B" and another one to receive data from "B" To "A".
So, if one direction data transfer is required - we need two wires and if bidirectional communication is required, we need three wires.
Usually, we use a shielded wire (one internal conductor and external shielding or, for bidirectional communications - an internal twisted pair of conductors and external shield).
Usually, the shield is connected in both equipment sides, but in some cases, there are who prefer to connect only one side and connect both sides GND pins directly to the aircraft ground.
Any how, there must be galvanic connection between the two GND pins of both equipment.
Ariel Arielly
RV8a 4X-OAA
Israel
 
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Just wait

If you think Garmin’s grounds are confusing, wait till you get to the elt. The elt RS232 grounds really confused me. But whatever, I seemed to have gotten it right because the elt past the initial turn on test and said it was receiving good rs232 nav data.
 
If you think Garmin’s grounds are confusing, wait till you get to the elt. The elt RS232 grounds really confused me. But whatever, I seemed to have gotten it right because the elt past the initial turn on test and said it was receiving good rs232 nav data.

Yeah I puzzled over that a bit but the response from Ariel really helped.

I guess the reference ground for the ELT is the aircraft ground. So long as the unit sending the data was also grounded to airframe I assume it would be fine.
 
Yeah I puzzled over that a bit but the response from Ariel really helped.

I guess the reference ground for the ELT is the aircraft ground. So long as the unit sending the data was also grounded to airframe I assume it would be fine.

As Ariel reiterated, RS232 needs a ground to function -- one signal wire for one way communication (like the ELT) or two wires, for two-way, PLUS some ground reference, either a explicit ground reference wire, a "shield ground" tied to the connector shells, or perhaps power supply ground.

A reference ground tied to the airframe is probably the most unreliable. Yes, the airframe is grounded, but it's not designed to be an electrically-bonded ground path (at least not in RV's I've seen) and and give very spotty electrical performance.

Maybe, you can get away with running a nav light or landing light return through the airframe, but it's never a good idea to use the airframe as a signal ground reference; too much potential for degradation and wild swings in return impedance back to the box on the other side of the RS-232 line. Just run the ground when you run the signal! Potential problems eliminated.

I will work likely in the beginning, and maybe forever, but YMMV significantly. This is the same reason why we run dedicated power returns back to the single point ground point and not back through the structure.
 
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