rs485 to ethernet networking

[terk]

Hey guys/gals, I am relitively new to the forum, but have already received a world of knowledge on networking. However, I am in need of advice on connecting 4 temp. Controllers to my entire network. I have already been able to connect my Honeywell HC900 to our network, but now I need to get the other 4 UMC3000 connected using rs485's. I was thinking about using an rs485 4 port to ethernet hub, but was werry on spending the cash until I asked y'all if this would be a wise idea or not. What are the difficulties with this, can I cascade my rs485's and than only have to get a one port rs485 to etherent hub? Hope you can help.

Thanks in advance,

Terk    

[MJF]

At my last employer, we did all sorts of network testing. We used Cisco RS-232 terminal servers to control a lot of hardware remotely and they worked quite well. Basically, the terminal server is a telnet server that you can connect to which passes the data traffic between the RS-232 device and your PC. The servers we used were 1U 19" rack-mount devices with 12 or 24 RS-232 ports. Each device port had it's own IP address associated with it so to connect to a given device, you simply opened a telnet session to the associated IP address.

So I think that your plan is pretty sound as long as you can connect to the RS-485 server device with the right protocol. Most of these things use telnet so you need access to a telnet library and an understanding of the device's commands and responses. If you are using a recent version of Labwindows/CVI or Labview, I believe you are covered on the telnet library. If you have another development environment, you may have to go find a library. I do recommend using a third party library if you don't already have one as the telnet protocol is pretty complicated and you would spend a lot of time implementing it.

[terk]

Hey thanks Martin. We will give it a try.

[JasonSmith]

Hello Terk,

National Instruments also offers a line of Ethernet connected RS-485 ports, which you can find at this link.  This are a bit different that the solution previously described, in that they will appear as typical COM ports on your computers rather than a telnet server.  The one caveat is that each RS485 port can only be used by one network user at a time, so the port may not be available when you try opening it.  I am guessing that this would be the case with the terminal server solution previously mentioned as well.

If you need many computers to access the same sensors simultaneously, you will probably need to read the sensors from one computer, then use that computer to publich the information on the network.  This takes care of the issue that a port can only be used by one computer at a time.

As far as connecting all of your sensors to one RS485 port, this is really dependant on the sensors themselves, and whether they implement some form of multidrop protocol to allow you to communicate with them on the same RS485 bus.

Good luck!

Jason S.
National Instruments

[terk]

How do I find out the address on my umc800. I am having a lot of trouble getting anything to communicate with it and I have no idea how to find the address for it. Also, I have a 4-port rs485 to ethernet connection that I am trying to use and thus far I can't get any of my devices to respond back to the router. Any thoughts? All the terminator resistors are in there on both sides of the lines, even though some of the lines are only 5 feet long.

thanks,

Terk

[JasonSmith]

Hi Terk,

As I mentioned there is not a single standard used by RS-485 devices in order to implement their multi-drop communications.  If you do not have documentation describing how to communicate and address the devices, you should contact their manufacturer for assistance.

Jason S.
National Instruments