RS485 connections

[Cat-Camera-Man 0]

Hi,

I have a PTZ connected with two wires from the RS485 adapter on the computer. This connection works fine. Now with a second PTZ camera, I have a problem: Looking at drawings on how to connect the second PTZ, brings up a thought. From the adapter A goes to A on the camera and B to B. Drawings show you should/must connect to A and B on the first camera, then go to the second. Well, if you look at the drawing, what is the difference in connecting both cameras at the adapter? In other words, there would be two wires to the first camera and two wires to the second directly off the RS485 adapter. Will this work?

 

R.T.

[survtech 0]

Most manufacturers don't recommend running separate control lines to multiple PTZ's from a single port. The problem is that the data signals can reflect back around the multiple data paths and cause intermittent operation.

 

There are two ways to run multiple PTZ's off a single RS485 port:

1. Daisy chain them by running your twisted-pair data line into one PTZ, then out to the next. Conventional wisdom says you should only have the terminator enabled on the last PTZ in a chain, but we typically leave all terminators on and daisy-chain up to three PTZ's with the termination turned on with no problems (makes a 33 ohm load).

 

2. Use an RS485 "data distribution unit" like the Pelco CM9760CDU-T or the Kalatel KTD-83.

 

With each method, your system must be able to send data to multiple addresses on a port and each PTZ must be set to a different address.

[Cat-Camera-Man 0]

That is what I had thought would be the correct way to connect them, but was not sure. Thanks for the answer to confirm. About two months from now, when the snow and cold are gone, I will work on that second camera installation.

 

R.T.

[Soundy 1]

Despite a "star" topology being "not recommended", I have done it, and/or seen it done on sites I've "inherited", on numerous occasions, and never had a problem with it. Look at it this way, you have nothing to lose by trying - it will either work or it won't, and if it does, it may save you some wiring.

[Cat-Camera-Man 0]

Soundy,

When I can find some time, I will do a "test connect" and see what, if any, problems I come up with. If it does work it sure would save some work in not having to climb a tower, dig into the current cameras wiring, and so on. It is worth an attempt.

 

R.T.

(P.S. I like the "bunny." Sure it is not a kitty kat?)

[survtech 0]

Then again, just pick up a Kalatel KTD-83 code distribution unit for a bit over $100 and you can run separate lines to each PTZ.

[vk5ztv 0]

Despite a "star" topology being "not recommended", I have done it, and/or seen it done on sites I've "inherited", on numerous occasions, and never had a problem with it. Look at it this way, you have nothing to lose by trying - it will either work or it won't, and if it does, it may save you some wiring.

 

If you have only 3 devices (camera - pc - camera) you don't have a star - you have a daisy chain with the PC in the middle. In this case you turn on the termination on both cameras and it will work fine.

 

As soon as you add a 4th device, don't connect that one direct to the PC; you then *do* have a star which *will* cause problems for RS485 comms - I know from personal experience and having to go and rectify other people's bad installations.

 

National Semiconductor have some RS485 application notes that are quoted by just about every other manufacturer of RS485 equipment (and are therefore considered fairly authoritative when it comes to the right way to do things). Worth reading and understanding.