URGENT : connecting over 200 sensors HELP ??

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Hi,
I am :? doing a project in which i need to
1) Connect over 200 MCU's :idea:
2) Each connected to two sensors.
3) A master which will communicate with all of them and collect data from them.
4) Protocol should have error checking and correction(correction can be left).
Project details:

In my project protocol should be such that it could carry data upto 750 Meters :cry:

Forget to ask Q. :arrow: Which Protocol should i use .

Thanx and Regards

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What is the application?

MODBUS should be suitable.

Leon

Leon Heller G1HSM

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thanx for your early response.

I have to put sensors in a building for temprature monitoring and water level monitoring of water tanks.Which are getting being heated and control the temprature of the heaters.
thanx and regards

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I agree that modbus should be suitable.
Build a small controller board with temperature and level sensor (and any other signal required) and a RS485 interface.
Each one will have a discrete address from 1 to 255 as modbus slave.
Using a central computer with one or two RS485 interfaces (optionally you can divide your network) will be very easy to scan these devices. The scan rate can be up to 30-40 devices per second, so a 5 seconds refresh cycle is possible (or 2.5 if you use 2 parallel networks)

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How many RS485 devices can typically be wired together? I didn't think you could do 200 - maybe I'm out of touch with the current transceiver crop.

Mike

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Quote:
I agree that modbus should be suitable.
Build a small controller board with temperature and level sensor (and any other signal required) and a RS485 interface.
Each one will have a discrete address from 1 to 255 as modbus slave.
Using a central computer with one or two RS485 interfaces (optionally you can divide your network) will be very easy to scan these devices. The scan rate can be up to 30-40 devices per second, so a 5 seconds refresh cycle is possible (or 2.5 if you use 2 parallel networks)

Thanx for your response.I know about rs485 but i dont know about modbus technology.
Is there any library available on this.
Can I do this project with 2 parallel TWI networks.
I am not quite sure about distance of TWI interface

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snarflemike wrote:
How many RS485 devices can typically be wired together? I didn't think you could do 200 - maybe I'm out of touch with the current transceiver crop.

Mike

There are 485 transceivers with a 1/8th unit load to allow up to 256. Maxim has quite a few parts with 1/8th unit load.
http://para.maxim-ic.com/en/search.mvp?fam=rs485&tree=master

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Obviously, the distance needed and the relative simplicity of the "modules" (1xMCU + 2xSensors) would probably push me towards a RF setup, something using the XBee OEM modules or something...

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UNiXWHoRe wrote:
Obviously, the distance needed and the relative simplicity of the "modules" (1xMCU + 2xSensors) would probably push me towards a RF setup, something using the XBee OEM modules or something...

I wonder if it wouldn't be cheaper than the cabling, too?

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divgup wrote:

Can I do this project with 2 parallel TWI networks.
I am not quite sure about distance of TWI interface

Probably, but not easily. I2C was designed for short distance comms.

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look into AS-i

Actuator Sensor interface

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What kind of environment should that run in? Factory / one huge hall / buildings with lots of steal and concrete / other power devices?

In a Scada project I was working we had to use fibre optics to connect the sensors in-house and for 20m distance, because each time a metro appears in that station the RTUs (sensor interfaces) resets.

If you have big heaters or better big pumps you need a good EMI shield; any long copper tends to become an antenna :evil:

On RS485 you will probably end up with optical isolated drivers on each node. 750m is definitively not a peanut!

If there is open-air environment cable to be used only think about fibre optics. Lightnings may kill your copper cable based system faster than anything else.
Knut

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Using kabels also helps to provide power to the sensors. Using 200 small wallwarts is probably not a good idea.

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Hmmm,

Inside a building, lots of walls, RF might be hard pressed, new cabling could be horrendously expensive ... so how about using the existing power cabling. If I remember correctly X10 was only for ON/OFF type command operations? Was it Lonworks or something like that that added two way comms. Old memory cells need replacement!

Cheers,

Ross McKenzie ValuSoft Melbourne Australia

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Quote:
What kind of environment should that run in? Factory / one huge hall / buildings with lots of steal and concrete / other power devices?

In a Scada project I was working we had to use fibre optics to connect the sensors in-house and for 20m distance, because each time a metro appears in that station the RTUs (sensor interfaces) resets.

If you have big heaters or better big pumps you need a good EMI shield; any long copper tends to become an antenna

On RS485 you will probably end up with optical isolated drivers on each node. 750m is definitively not a peanut!

If there is open-air environment cable to be used only think about fibre optics. Lightnings may kill your copper cable based system faster than anything else.
Knut

Hi should i use Serial 2 Ehthernet modules to connect the 255 different modules.In that way each of them would get a seperate IP address.Although i am bit confused about which Network Topologyu should i follow.??

Anyone has tried this before??

I appreciate that everyone is ready to share there thoughts and experiences

Thanx and Cheers

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Without knowing what you're going to install this in, it is hard to give a recommendation. If we assume its a building of some kind with a number of floors I would suggest a hybrid of ethernet and RS485 using Modbus. RS485 for the particular floor and a ethernet to modbus gateway for each floor. Each floor connected by CAT5 or a fibre optic backbone (most modern buildings have this already). That way faults on a RS485 bus are constrained to one floor. I can tell you trying each device one by one to isolate a fault on a bus with many devices gets laborious!

As Knut mentions, the exact method differs depending on the environment - outdoors, lightning protection is a consideration, whereas indoors it is less of a consideration, although some surge protection is always worthwhile.

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divgup wrote:
thanx for your early response.

I have to put sensors in a building for temprature monitoring and water level monitoring of water tanks.Which are getting being heated and control the temprature of the heaters.
thanx and regards

The application is to monitor water level and temprature of overhead water tanks.The cable will be open to the sky but some arrangement can be made to protect it.They all will be on one roof.
I was thinking of connecting serial data from Slaves which are also connected to sensors by Serial to Ethernet modules.And then writing an applcation on the PC side to ping and collect data from different modules.

Can i use bus topology because if i use start topology i would have to buy a router with 255 sockets.
I am doubtful on using bus topology because it is said to be error prone.Please suggest what topology should i use Or this method is useul or not.

Thanx and Regards

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As a possible alternative, check out the DS18S20 temperature sensor from Maxim.

If you think education is expensive, try ignorance.

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I saw an application using ModBus/RS-485 to commuicate with about 200 temperature stations, with about 350 sensors installed in a huge supermarket. Definitively more than 1000m of cabling. Since I had developed some of this products, I will use ModBus for your suggested application.

Anyway, I agree with others that more information would be needed about your exact application (industrial environment, existing wiring, different plants, etc).

Guillem.
"Common sense is the least common of the senses" Anonymous.

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With ethernet, understand the difference between hubs, switches and routers. Using CAT5 with ethernet is star by definition. A large network would normally consist of hubs, switches and routers rather than one large 'router' with 255 connections. As such, with ethernet you no longer run a bus - it is star or actually more like a tree.

Using a hybrid solution like I suggested earlier, the RS485 busses have a small number of slaves - busses are not necessarily error prone, just that when you have a large number of nodes, they become difficult to debug if something does go wrong.

Having a PC control a large system is risky at best, I'd suggest you use PLC's to manage local control and sensing and have network of these. This is the usual method you'll find in building management systems. That way the PLCs manage the critical tasks and the PC just grabs data from them. If the PC fails, the system still operates.

Note that we can only give suggestions, it's still up to you to make a decision and to bear the consequences. Asking for 'urgent help' for what looks to be a commercial project is to me, professional suicide. Do your research - find out what has previously been done in this application and what industry standards there are.

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That type of application, energy meters in apartment buildings, seems to be the flagship app for Zigbee and wireless communication, repeaters, and the like.

With inexpensive wireless sensing nodes at about
US$20-30 it won't take much wiring between nodes for the wireless to become cost-effective -- e.g., about 30 meters if wire is $1/meter. In practice perhaps a lot less given the labour of stringing the wires.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.