CHEAP Serial bus, control 200 devices, 50 meters

I'm asking on this group because you folks seem to be more aware of the lower cost options. I wouldn't mind some outside-the-box ideas.

I need PC control/automation of: 200 machines Each machine needs 16 logic inputs and 16 outputs Line lengths may be 50 meters
I want a serial bus to connect the i/o devices at each machine with the PC. This is to reduce the wires needed, lower costs and simplify design.
It is not a noisy environment. No welding, for example. I don't need more than 100kHz speed, and might get by with even less. I don't need environment protection. The devices can be just a circuit board. It's indoors, not a washdown area. I don't need super high reliability. It's not a life support system. Cost is a major concern.
I've been looking at some industrial bus options. Either they cannot handle 200 devices, cannot handle 50 meters, or are very expensive (x200). But, there are so many, I don't know them all, and maybe some can be modified to do what I need that I'm not aware of.
Can anyone offer a suggestion or alternative?
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Brook:
The RS-422/RS-485 stuff is probably what you are looking for. RS-422 is point-to-point and RS-485 is multi-drop. The interface chips are readily available.
-Wayne
Brook Stevens wrote:

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Except not every RS485 transceiver can handle 201 nodes. They can handle 32 Unit Loads (UL), most are 1UL/transceiver or 1/3UL/transceiver, but some have a lower UL. The LTC1487 can handle 256 devices.
Patrik

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RS-485 sounds great. But, then what protocol should the devices use to communicate with the PC? The PC needs to address one out of 200 devices and tell it to set it's inputs, or to send it's outputs. I find lots of industrial bus options out there that have their own a protocol.
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How about a serial port per unit? They have, at least 10 years ago they did, concentrators that will run 16 ports remote from the CPU.
This was IBM hardware at the time, but I think the same is available for PC's. Unix has no trouble accessing 250 ports. Otherwise look at terminal server types of products.
Then a PIC based controller, with a serial connection, depending on your hardware.
Device independent, in that one unit does not depend on another to function.
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Bear in mind that using a PC as the host controller can allow for more than just one interface BUS. While each technology (485, 1-wire, etc) might have device, speed or distance limits that doesn't mean you can't use more than one.
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How cheap did you have in mind?
100KHz? do you mean Kbit? KBaud?
Are you looking for Relay outputs? optical outputs? What sort of input? Digital / Analog?
Yes RS485 would appear to be the way to go. Try www.rs485.com

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William Richardson wrote:

You're right, that was unclear. I meant KBaud, Kbps. I realize the protocols will have some overhead, and that's fine. I could maybe get by with 9600bps.

Logic level.

Thanks for the suggestion! That's very interesting.
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Have a look at my site www.brightan.com This system is capable of 5*128 nodes with each node having 128 devices (single input or output or complex device like a RS232 port). This is all CAN (Controller Area Network) based. You can even download new firmware to a node across the network. Although the main idea is lighting control I have designed the protocol and the display software to be expandable to whatever you want. I do not currently offer a 16 input and 16 output module but it would be trivial to create one.
Steve
--
Steven D. Letkeman BSc.
President - Zanthic Technologies Inc.
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Have a look at this...this stuff has been used for just about everything you can imagine, and if done right...is bullet-proof...
It's not shrink wrapped...but it extremely flexible and you can do whatever you need with it. There is also an extremly active user community...
www.jlcenterprises.net/Products.htm
It was originally developed for the Model RR community, but it's been used for just about everything...and you can get it in kit form...or completely assembled...or just buy part...so cost can be contained...depending on your situation. Also will run on very old & slow machines...on dos...or...on windows if you like...
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