Need to transmit video/data over long cable

Jay Kickliter

I'm working on a school project where I need to transmit serial and video over a ~60' umbilical. My original idea was to put an old Mac Mini I have on the remote device to handle the interface, and send all telemetry over a CAT-6 cable via ethernet. But, I'd really like to keep it simple and eliminate the computer, and handle the serial and controls with a Propeller chip, and send the video separately. However, I really like to keep it all in a single CAT-6 cable, using the bare wires, not using ethernet.

Two questions:

Using a plain CMOS video camera, something like this, can I send that signal over twisted pair without a buffer? Or will I need coax?

And what is the max length for TTL Prop <-> Prop serial comms? Will I need a RS-232 or RS-485 converter?

Thanks

schill

I have done similar things and this has been my experience.

For the data lines, I would use RS485 in cat5/6 cable. I have used it to about 2000' in testing at 9600 baud and never had issues (my limitations came from also running power through the same cat5 cable). You should be able to go faster, even at that length. I usually use a separate pair for transmit and receive (4 wires total) instead of sharing a single pair. If you've got the wires available, I've found it really simplifies things. RS485 driver/receiver chips are inexpensive so there's not really anything to prevent using them.

The RS485 drivers will typically also give you some electrical isolation/protection between your devices which is not a bad idea either.

I have run composite video over cat5 for runs of at least 100 feet without thinking it was too bad (I don't remember if I threw any resistors into the mix to try to match impedance). I've had longer runs as well but not up to thousands of feet like for data.

I have used this setup for remote monitoring of a camera. Video was recorded at the camera itself and the composite video over cat5 was just used to monitor the status, make sure the camera was pointed in the right direction, etc. I was not too concerned about quality (it was recording HD at the camera end). The camera was being controlled via the data lines described above (but not for 1000 ft).

I suggest you try it and see what you think. You can try the video without any electronics being involved - you just need the right connectors.

Phil Pilgrim (PhiPi)

Google video balun cat5. A balun converts an unbalanced (i.e. ground referenced) signal to a balanced signal suitable for transmission over twisted pair cable -- and back again.

-Phil

Jay Kickliter

schill, thanks for the info. I was hoping I wouldn't need to use RS-485, since I'm not familiar with it. But I guess it's just a matter of using interface chips? Also, I'm surprised that the video can work at that length.

Phil, I did a search, and couldn't find much about how they actually work. Have any idea how many of the conductors they use? I also need to use the same cable for data.

Phil Pilgrim (PhiPi)

Jay,

A balun is basically a transformer, in this case to convert an unbalanced 75-ohm signal into a 100-ohm balanced signal and back again:



It will use one pair of your CAT5/6 cable. The other three pairs are free to use as you wish for data. I also recommend RS422/485 for data transmission.

-Phil

schill

For RS485:

I like the chips that have separate transmit and receive connections for point-to-point communications (full duplex). They require 4 wires (2 pairs) instead of just 2, but you don't have to worry about switching modes. Other drivers will work with 1 pair, but you have to control the mode (receive or transmit) at either end (these are half duplex).

An example of one of these chips is the MAX488 (available in dip form).

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MAX488CPA%2B-ND

Note that this chip is from Maxim and is not the cheapest available but it's a good example. It runs 3.28 a piece from www.digikey.com (or 2.18 each if you want 25).

Here is the data sheet:

http://pdfserv.maxim-ic.com/en/ds/MAX1487-MAX491.pdf

If you look at Figure 2 on page 8, you will see what is pretty much the entire circuit that you need. there isn't much to it. Note that these are 5v chips so you would need to interface appropriately to the Propeller. You would probably want to put capacitors on the Vcc pins as well. The resistors labeled Rt in the diagram are terminating resistors, typically 120 ohm if I recall correctly. You may be able to run without them but they won't hurt - especially for long runs - so you should include them.

There are 3.3v chips out there as well, but I don't know if they are available in DIP format. If surface mount is ok, you'll have a lot more options. The MAX3488 and MAX3490 are 3.3 volt and available in DIP, but www.digikey.com does not stock them.

I've used chips from other manufacturers (Maxim is almost always the most expensive) but I don't have them in front of me and don't remember what they are. Maybe STMicroelectronics or Sipex?

Jay Kickliter

schil, you sure saved me a lot of time, thanks. It looks a lot easier than I had expected. I do everything surface mount, so I'll look for a 3.3 V chip.

Phil, same to you as always. Last question(s), from the best that I can tell, for composite <-> twisted pair, I need 75 ohm unbalanced and 100 ohm balanced. Think that's about right? I don't see any board mount baluns on Mouser or DigiKey. I can only find them in finished CCTV products. I read all about them, but still don't quite understand the difference between balanced and unbalanced. I feel like an idiot. I don't quite get the difference between a signal line and a ground line, and the two lines coming off the secondary of the balun. Actually, while typing this, I think I just saw it (will leave for future reference anyway). Is it that ground is ground, and any noise the ground line picks up gets transferred to the signal line since ground is used as a reference? And the balanced line, there is no reference? So since both lines are out of phase, but noise is in phase, the noise gets canceled out when re-combined to an unbalanced signal? Kind of like an instrumentation amplifier? Please ignore all the the question marks, I'm thinking aloud. So I think I get balanced now, but still don't understand unbalanced, the complete opposite as of yesterday.

So I think I finally understand why both of you recommended that I use RS-485. In the object we're controlling, there's three fairly large DC motors, and several lead acid batteries. RS-485 is a differential signal, so if we pick up any noise, it gets canceled out. As far as using a balun for the video, it looks to me like it gives the same benefit of using a differential signal, but isn't actually differential. Still confused there.

Anyways, Phil, thanks for all the help you've given me, past and present. My goal is to have as much power over 'trons one day as you:)

Phil Pilgrim (PhiPi)

Jay,

Yeah, I see the problem with finding PCB-mounted baluns. You could also try an active circuit approach. This article contains some links that might help:

http://www.epanorama.net/links/videocircuits.html#differential

Here's a link to a Maxim chipset datasheet that might be the simplest approach:

http://datasheets.maxim-ic.com/en/ds/MAX9546-MAX9547.pdf

Here's a detailed article about differential video:

http://electronicdesign.com/article/analog-and-mixed-signal/for-better-analog-video-try-differential-signaling.aspx

-Phil