Data protocal between two devices? - Page 1
Forum section: electronics / projects

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 19, 2011, 03:35:49 pm
content: I'm working on an antenna tuner project. I will have a box with a micro in it to measure RF forward and reverse power. It will also switch latching relays to bring in and out inductors and capacitors in an L type network to tune the transmission line to an antenna. This will be remote with no local display or controls. For me only 12-20 feet away from the base unit, but the possibility is that someone would like to build this with it remoted maybe 100 feet.Power requirements at the remote will be 5V with maybe 300mA peak during relay switching and under 50 mA (hopefully far under) for normal running. It would also be great to allow remote power up of the link, as the latching relays will hold the setting with no power. This shouldn't be hard with a command to "get ready to shut down" and then just killing the 5V power over the line. There is probably a more elegant way to do it.The main device with have a display and buttons that controls and reads data from the remote unit. It will also supply power to the remote unit from the wires going out for data.I've worked a little with RS-232, not much with implementing the "real" version, just usually TTL level RS-232 for micros. Is there a good resource for reading the advantages/disadvantages of various protocols and interface types? I could see where going with RS-232 would allow for someone to replace the main device with a PC if they like. These data lines will be running near coax, possibly containing up to 500W of RF energy. Much less in my use, but trying not to design in limitations when I don't have to.

author: Rufus
author_rank: Super Contributor
timestamp: May 19, 2011, 04:22:47 pm
content: For talking to a box which might be stuck on top of an antenna mast I would want something robust as hell.I would go for an opto-isolated current loop. If you search for LDM35-S you will find data on an ancient Burr-Brown RS232 to isolated current loop 'modem'. This 'modem' is just an opto-isolator and a handful of discreets. I don't suggest you use this device but the data tells you what is capable of and it is easy enough to create your own circuit to do the same thing.Run the loop over twisted pairs and put extra protection on it, possibly GDTs if you want it to survive lightning strikes.

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 19, 2011, 06:12:40 pm
content: The opto-isolated differential RS-232 is where I was thinking. Although GDTs were new to me. Didn't even know what they were. I guess the simple explanation is a neon lamp.For others in the same boat. Gas Discharge Tubes as devices that have very high resistances at normal operating voltages. When hit with a surge from a lightning strike or similar, they will dissipate that energy across themselves to keep it from arcing in other places. Looks like they run $1-2. Interesting.Would these be typically installed just between all data and power wires going into a device?I'm thinking about a CAT5 data and power cable, with 2 pairs of +12v and GND. 2 pairs for differential send and receive. Two GND and Two +12v get tied together, so I have 5 GDTs between GND and all other lines?I'm gonna have to do more reading on these. It would be cool to have a board design that allows them if needed, but you don't have to include them if used in an isolated environment.

author: Rufus
author_rank: Super Contributor
timestamp: May 19, 2011, 07:08:07 pm
content: Quote from: sacherjj on May 19, 2011, 06:12:40 pmThe point of GDTs is they can absorb kilo amp+ surges (but not many of them). The striking voltage is maybe 100v upwards so you still need additional protection like series resistance and tranzorbs. You probably have a 200v GDT across your phone line where it enters the house.Where to put them depends on your circuit. If you go for current loops with opto-isolated receivers there isn't much possibility of generating large differential voltages on the twisted pair so you don't need them on the receiving end but do need to protect the transmitters from large common mode voltages with respect to ground. Power supplies need protection with respect to ground both ends, or maybe you use an isolated dc-dc converter and the required protection is more like the current loops.

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 19, 2011, 07:12:45 pm
content: Thanks, Rufus. Good stuff. I've been wanting to learn more about protection for this and sensors, etc.For others interested in GDT this is a part advertising, but also informational page I found at Digikey.http://www.digikey.com/Web%20Export/Supplier%20Content/Bourns_118/PDF/Bourns_FirstPrinciplesGasDischargeTube.pdf?redirected=1I love learning new stuff.

author: Zad
author_rank: Super Contributor
timestamp: May 19, 2011, 09:24:53 pm
content: There are quite a few designs for ATUs around, both continuously variable and switched, they are understandably popular in the amateur radio community and are often published in ham magazines. Very few of them have much in the way of lightning protection beyond the inherent isolation in the relays etc. Why? Well because if you get a strike close enough to induce that much power in the antenna system, you are well and truly f****d anyway to be honest. Having RF so close shouldn't be a problem with the right design, with sufficient RF filtering on the power and control lines.By the way, it is more usual to have the ATU immediately after the TX/RX so that transmission line characteristics are compensated for too.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 20, 2011, 02:01:51 am
content: The idea of having the ATU at the bottom of the antenna,instead of at the operating position is to allow theconvenience of using flexible coaxial cable without incurring losses due to operating these cables at high VSWRs.There are a number of commercially made products which do the same job.I would suggest the OP have a look on QRZ.com,as similar questions have been asked on that forum.RS232 only needs 2 conductors,so you could possibly run 2 lengths of thin coax,with the shields commoned.I don't know if that would be better than twisted pair,but it's worth trying if you have plenty of coax hanging around.You can certainly get shielded twisted pair cable,we used it for RS485 control of some ISM Transmitters.VK6ZGOPS:-RS485 may be better than RS 232 for your application. I found a lot of info on both when we were working on thesystem I referred to above.I'll see if it is on this PC,& will post the URLs if I have them.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 20, 2011, 02:16:16 am
content: Here are two-http://www.bb-elec.com/technical_library.asp(These people are very knowledgeable,especially about odd little niggles you may be having)http://www.radio-electronics.com/info/telecommunications_networks/rs232/eia-rs232-c-d-standards.phpI don't know why I'm posting all this data crud,I'm an RF person!VK6ZGO

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 20, 2011, 03:13:10 am
content: I've seen many ATU systems and will be using an L network, with 8 inductors and 8 capacitors to switch in to 256 values of each, with a toggle to switch the C part to either side (to hit the lower impedance side of the match.) I'm borrowing from a few designs, but want a little different interfacing to the micros.As far as the lightning protection, I was debating if that was the case. Being basically vaporized anyway.My own application is to run coax to a choke balun, to feed balanced 50 ohm in and then hit a 450 ohm ladder line up to an 80 m dipole. I want the tuner located a ways away to be at the coax to ladder line junction. The loss of coax for the distance I'm running up to the dipole just won't cut it for QRP.I'll be building this in sections and learning each way. Building another's ATU design gets you an ATU. But building your own and working out the calcs for the tuning makes you really understand the process involved.I also need to read more about high SWR protection for my transmitter while I'm playing with the tuning and programming. I figure that some "bad" dummy loads of 20 ohm or 120 ohm, maybe with a little crazy reactance, will let me "play" with the tuning.Quote from: vk6zgo on May 20, 2011, 02:16:16 amI'm trying to get there. I've built an 80m SSB and 40m CW rig. Learning analog along the way. Both rock bound kits. Once I get some of the basics down, I want to build up some rig sections from scratch. Just go ugly bug on a board. I have a bit more work to get the test equipment I'm building done before I go there.

author: RayJones
author_rank: Frequent Contributor
timestamp: May 20, 2011, 10:49:32 pm
content: The discussion has seemed to have morphed into the physical interface between two devices, as opposed to theprotocolbetween devices....You can carry the same protocol over many different physical methods (just look at TCP/IP).I'm not dismissing the need for a reliable physical connection, but at the moment the discussion has not matched the title!Is there any merit in following the VISA commands used to control sig gens, power meters etc?There may not be any commands for antenna tuning, but maybe they do...This is at least a form of industry standard, and learning the coding of that protocol will be useful should you want to do an "automatic bizzo what's it" with other RF test equipment...

author: vk6zgo
author_rank: Super Contributor
timestamp: May 21, 2011, 03:23:11 am
content: Guess you're right Ray,the discussion has been pretty wide-ranging,but the subject in the OP'stitle was hardly mentioned'In my case,as a hardware oriented person,the physical stuff like RS232,etc,is the area I knowa bitabout.Personally,I'd be happier with every function having its own separate control cable,with big thumping APO stylerelays doing everything,like it was done in the old days!VK6ZGOPS:- sorry about all the edits,touchpads & I don't see eye to eye!

author: Rufus
author_rank: Super Contributor
timestamp: May 21, 2011, 05:47:10 pm
content: Quote from: RayJones on May 20, 2011, 10:49:32 pmIt didn't morph. The first post talked about line distances, power supplies, RS232 and the electrical environment. The discussion hasn't matched the title from the start. It was a poor choice of title.