CHEAP Serial bus, control 200 devices, 50 meters



RS422 is current loop and therefore only usefull for peer-to-peer connections. RS485 is a differential interface which may be implemented as a half duplex bus or a separate transmit and receive pair for a full duplex peer to-peer connection.
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Nico Coesel wrote:

Both RS-422 and -485 are differential voltage. A transmitter enable is optional with RS-422, but required on RS-485. I wrote of RS-422; that's how the project started. We switched to RS-485 when we built the prototypes.
Jerry
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Rene Tschaggelar wrote:

If a strict master / slave relation ship is OK ... why not.

Oh ... I just hit the wrong key. RS485 is the correct one.

8 bytes of data ... there is room for are 32 input and 32 output signals and this is more than requested.

Longer messages are handled by CANopen ...

No ... there is a real confusing set of incompatible layer 7 protocols used by the car industry.

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Brook Stevens wrote:

There are a lot of serial buses- Profibus, Modbus, Bitbus etc. etc. Most limit the number of drops- ususally 7 bit or less addresses, but more often the drivers have a lower limit anyway. So you'll probably need to partition the system into several buses.
You'll probably need to do that anyway, to get anything like a realistic speed of response- these buses tend to poll around the connected devices, and with a well- pouplated bus you vcan get significant delays.
Paul Burke
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Brook Stevens wrote:

A scan time of 1kHz is possible with Profibus DP (12Mb/s) under QNX6.3 and ~32 devices. In order to handle 200! devices you need at least 2 DP masters ... but the scan time is much higher.
The fieldbus Interbus is able to handle up to 255 devices by one master ... but the scan time is then > 50ms. It's going to be faster if additional masters running bus segments with less devices. Up to 4 controllers are supported in one PC under QNX6.3.
A faster scan time is possible with our new EtherCAT (100Mb/s) master (available end of Q1, QNX6.3) ... the realistice maximum scan frequency is then 5-10kHz. One master can handle up to 65k devices :)
Best Regards
Armin Steinhoff
http://www.steinhoff-automation.com

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Look for ethernet I/O boxes. Something like this (I just found it with Google): http://www.hw-group.com/products/io_controller/index_en.html
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If you're going to design this hardware yourself you could use any simple microcontroller and serial support chips. BAre bone it can be made for about $1/point or I/O pin. RS485 ,even RS232 could be used fro serial communications. What isn't said is speed/throughput of the network, master/slaves,etc.Use of SSRs for machine I/O ? etc. Include that with the micro and you could do it for about $200 per machine as I've been there done that. Jay
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j.b. miller wrote:

A fieldbus related IO module costs less :)
Armin

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j.b. miller on 2/1/05 wrote:

That sounds good.

I'm thinking speed is not a major factor. The PC (ie: the module that the PC is connected to) could be the 1 master, and every other module can be a slave.
SSRs would be fine. As long as I can get TTL i/o at each machine, I can handle the rest from there.

I'd hoped I could do it for less if designing the hardware myself. Are you sure it would be that much?
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$10 for micro and minimal support stuff(xtal,c's,r's,etc) $5 /SSR * 16= $90 $10 PCB $10 case $10 I/O connectors $10 power supply stuff( c's and reg) $5/Input proection/level shift etc. *16 = 90
and that's just the rough cut for hardware !
jay
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Not sure what you mean by "100kHz" but in addition to the rest of the suggestions, you CAN run RS232 in a ring topology, where A talks to B and listens to Z, B talks to C and listens to A, and so on. Messages have a sender and recipient, and anything that isn't for "me" gets passed along the chain. You also have a "time to live" that is decremented whenever the packet is passed, so if it is a 30 device loop, you set that to maybe 32.. When it hits zero, the packet is dropped.
All the devices have to implement the protocol, and deal with dropped/corrupted packets, but it's a pretty nice way to go, in that at any point in the loop, only one machine is talking to one machine on a cable, so you get full bandwidth between all machines, at the expense of a short time delay to move the packets.
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Dave VanHorn wrote:

Sorry, I meant 100k bits per second.

Very interesting! Thanks for the idea.
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Brook Stevens wrote:

... and you mean realy "200 devices in a range of 50m" ?
Must some kind of NANO technology :)
Armin

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On Wed, 02 Feb 2005 11:32:32 +0100, Armin Steinhoff wrote:

What's wrong with having 200 devices withing 50 meters?
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keith wrote:

Let's see ... 200 drops in 50 meters ... on average, the drops are 25 centimeters apart. If the controller is in the middle and the bus goes out in several directions, the drops may get as far apart as a meter on average. That's still a pretty tight cluster of machinery/
Jerry
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On Wed, 02 Feb 2005 21:14:34 -0500, Jerry Avins wrote:

Not nano, though. I wuz thinking more "within" 50m.
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Jerry Avins wrote:

Lets see. 50m radius makes almost 10000m^2. You'd fit 100 devices in there with 10m distance each.
Rene
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Rene Tschaggelar wrote:

But how long will the signal wires be?
Jerry
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I've actually implemented such a topology several years ago. It has a huge overhead. You won't achieve full bandwidth either because you'll have to send all the packets through the first device. This divides the available bandwidth by the number of connected devices. Also, a small microcontroller won't like a burst of serial data and may drop a packet. To keep packet management simple, you will only want 1 outstanding packet per device. This dictates you'll have to wait for the acknowledge which takes a while because it has to travel through all devices. This take more bandwidth away because of dead time (especially when all the data originates from one source).
There is a protocol which deals better with these sort of setups. It's called TCP/IP.
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As I remember, when we did it, it worked pretty nicely. It was somewhere in the late 80's I think, but it worked well enough to reliably move money around. :)

Seems like a lot of overhead, I guess you could toss out the parts you don't need.
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