SL40 RS232 Signal Ground

Greetings,

I'm wiring up the panel, and noticed that the SL-40 has an RX, TX, and signal ground for the RS232 communications. The 496 only has a data out, and no "data ground". For that matter, none of the other serial devices have a signal ground either.

I'm guessing that most devices reference the data to ground internally, but perhaps some devices like the SL-40 leave the signal ground floating. My plan is to connect this signal ground cable to "ground" on the plane, and figure that all devices will be using that as a reference.

Anyone see a problem with this? If so, type fast, because I'll be wiring this up in a couple hours

Cheers,
Rusty

Not sure of the internal details of the SL-40, however, the install manual shows the serial ground carried with data in a shielded cable.

This implies that the SL-40 ground should be connected at the far end to a signal ground.

Nevertheless, if you have properly grounded your avionics and are using true RS-232C signal levels, you will have no problem just grounding this pin at the SL-40.

If, on the other hand, you are not using true RS-232C signal levels (e.g. the output from a handheld GPS), you may experience electrical noise problems doing this.

I know this may not be helpful, but I'd recommend following Garmin's recommendations. I'm hearing about problems in the field with instrument communications that may be electrical noise due to poor wiring.

Vern Little

Hi Vern,

Thanks for the thoughts. The sending device is a Garmin 496, and it will be received by the SL-40, Dynon D-180, and Digitrak-II.

I thought about calling Garmin to ask, since the only question is from the 496 to the SL-40. Surely they would be able to tell me how they want this, but then again, the SL-40 is supposed to be professionally installed, so they might not like the question.

For now, I think I'll just hook the "signal ground" to ground, and see how it works. Worst case, I'll just disconnect the SL-40, since being able to send frequencies from the 496 is not necessarily something I'd do anyway.

Rusty

I use the 396/SL40 combo and used the RXd input only with no ground connection, works like a champ!

Great to know Walt. Thanks.

Rusty (hates hooking up shielded wire)

Help: SL40 serial wiring?

Quick question: I'm going to connect my SL40 serial I/O to a GRT EFIS serial. The SL40 is wired with serial in, out, and serial ground. What do I do with the "serial ground" at the EFIS? Can I leave it open (not connected) or do I have to connect it to ship ground, or what?

(I never claimed to be an RS232 whiz, just that I have a firm appreciation for its simplicity and effectiveness .

RS232

Generally, you connect the serial grounds of the two ports together without connnecting to ships (or any other ground). The connection will find its own ground inside the boxes at the RS232 driver/receiver chips. So your link will have 3 wires between the boxes with no connections to anything else.

TX ------------------------> RX
RX <------------------------ TX
SerialGND -------------SerialGND

John

Serial Grounds

Since all of my avionics talk to each other via serial data lines, I created a single serial ground point, and routed them all togetehr, just to keep them all at the same potential. Seems to work fine!

Paul

Ok, I'm still confused. I don't see any pin definition on any of the GRT EFIS equipment that describes a place to connect a serial ground. I do understand creating a single serial ground point and connecting them all together, except the SL40 is the only piece of equipment that has a serial ground pin defined. More input requested .

Hi Scott,

I asked the same question here: http://www.vansairforce.com/community/showthread.php?t=9830

I ended up connecting the "signal ground" to the normal ship "ground", and everything works just dandy. That's what I'd recommend, though I think you could probably not connect the signal ground at all, and it would still work.

Cheers,
Rusty

The SL40 will serial gnd through the tray, which is common. One serial wire connection to the EFIS is all that is necessary.
Best,

Thanks for the clarification. Sorry I didn't find the previous thread. I think I'm going to leave it open on the EFIS side and see what happens.

Walt said:

I use the 396/SL40 combo and used the RXd input only with no ground connection, works like a champ!

Click to expand...

OK, here's the dope:

I'm a big fan of the SL-30/40 serial wiring scheme... i'ts bulletproof. On the other hand, the practical aspects are that not all avionics uses it, so interconnect is a bit of a compromise.

I had wired my GPSMap296 to a GTX327 transponder with a single shielded wire, hoping that the auto flight timer and auto standby modes would work... no dice. I talked to Garmin and they said that the GPSMap does not talk to the GTX327 (insert expletive here).

Today, I rerouted this connection to my SL-40 by removing the pin from the GTX327 and plugging it into the connector for the SL-40. Turned out the wire was the right length, so this was as simple as could be expected.

I tried the SL-40, and it worked with the GPSMap just by moving this pin.

I then also grounded the signal ground pin on the SL-40 by connecting it to airframe ground. The configuration worked as well.

So, connecting the GPSMap to the SL-xx with a single wire shielded cable grounded at one end, and connecting the SL-xx signal ground to airframe ground looks like it will work. You may get away not grounding the signal ground pin on the SL-xx to the airframe.

I have not flight tested this configuration, but I expect it to work fine.

**Caveat: the serial output of the GSPMap is buffered by an RS-232 converter board of my own design. This converts the low-level signals from the GPSMap to proper bipolar voltage levels, compliant to RS-232C. This provides more electrical noise immunity for interconnect, but I don't know if it's a factor in the GPSMap to SL-xx interface. Since the downloading of frequencies is not a flight critical function, I would not be too worried.

Vern Little