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Author Topic: RS485 to coaxiale 75 ohm schematic...  (Read 1372 times)
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France
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I guys,

I'm looking for a schematic to send/receive rs485 datas by an 75ohm coaxial cable (video HD-SDI).

If some one can help me about this stuff, I don't find it on internet.

The goal is to communicate with an Broadcast Video Camera to drive some parameters (iris, black, color balance,...).

Best Regards
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There is not much else to the circuit other than the 485 chip (Max485, SN75176...)

See post #7 of thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1270227591/12

Futurelec have breakout boards (http://www.futurlec.com.au/Mini_RS422.jsp) as do Sparkfun (http://www.sparkfun.com/products/10124) who have a schematic at the bottom of their page.

Some other links:

http://www.lammertbies.nl/comm/info/RS-485.html

http://en.wikipedia.org/wiki/RS-485 (talks about cameras)

http://www.maxim-ic.com/app-notes/index.mvp/id/763

The co-ax is not ideal. It should be twisted pair with and impedance of 120-150 ohm.
The co-ax may work for short distances. (http://www.bb-elec.com/tech_articles/cable_selection.asp)
Don't forget the terminating resistors.

That touches on the 485 electrics and protocol side but how your camera sends data/receives commands is up to you.

« Last Edit: July 07, 2012, 06:36:36 am by lemming » Logged

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RS-485 needs at least 3 conductors, I don't see how it will work with coax.

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Rob
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I thought rs485 only used a differential pair, no ground? I've only started playing around with it but I thought a common ground could actually be bad for it
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Do you have to use coax? The twisted pair as said before is what is specified for usually, you can always adapt it at the end along with those terminating resistors
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I doubt single-core coax will be suitable. The whole idea of differential pairs is that any noise affects both wires more-or-less the same. The coax would defeat that.
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A Coaxial cable if used to carry control signals cannot co-exist with Video and it would resuire two Very special connectors on the equipment being connected... as Coax uses different connectors (Grounded). At low < 100Khz data rates and short (under 10 - 20 meters) the line impedance isn't really an issue... Just terminate them by the book. The real issue is that proper termination leaves them @ 1/2 Vcc and thus cannot exist with regular connectors, You Could use Twinax... but since you would have to "Pull" the cable through the harness any way it might just as well be shielded 2 way wire or 2 conductor shielded wire... Much Less expensive than Twinax although I seem to remember 150 - 300 ohm Twinax... Mightily expensive though.

Doc
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GREAT DRAWING on RS485 implementation BTW Cogent and Comprehensive. Thank You (Not for me i've used it for years) A great illustration

Doc

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@ Nick

Shouldn't there be ~120 Ohm terminating resistors at either end of the bus?
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I think I left those off for a reason. In this particular case the bus is terminated at each end by the transceiver itself. If you have a long cable run, and the devices are connected somewhere in the middle, then yes, I believe the termination resistors are required.

Example documentation:

http://www.maxim-ic.com/app-notes/index.mvp/id/763

I quote:

Quote
One Transmitter, One Receiver

The simplest network is one transmitter and one receiver (Figure 6). In this example, a termination resistor is shown at the transmitter end of the cable. Although unnecessary here, it is probably a good habit to design-in both termination resistors. This allows the transmitter to be moved to locations other than the far end, and permits additional transmitters to be added to the network if that becomes necessary.
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Quote
Shouldn't there be ~120 Ohm terminating resistors at either end of the bus?
Normally that's the case but for short runs and low speeds I don't think it's absolutely necessary. The termination Nick has is called "fail safe termination", it's designed to force the line to a known state if neither transmitter is active.

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Rob
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