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kkdelabaca
Joined: 18 Apr 2003
Posts: 106
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 20 Feb 2008 17:29 How I can detect the 3 possible states in a bus RS485? |
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Hello,
I'm very interested in detect the 3 possible states in a bus RS485: Hi, low and High-impedance.
Some devices like the max485 can detect the level Hi setting up the level in the Receiver pin, but can not differenciate the state of LOW and High-Impedance.
How I can detect the 3 possible states in a bus RS485? It is posible?
Thanks!
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FvM
Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany
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20 Feb 2008 20:47 Re: How I can detect the 3 possible states in a bus RS485? |
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This is basically not possible. You could do it by an agreement, setting the high impedance state to "break state" by weak pullups, the state would change then to idle if a transmitter is activated.
Normally, the contrary condition apllies: You can expect that no bus participant will activate it's transmitter without sending data shortly. Thus idle state (lasting longer than one or a few frame periods) effectively equals high impedance state. However, if there are no clear rules who is allowed to send next (master/slave protocol, token passing), this wouldn't mean anything.
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kkdelabaca
Joined: 18 Apr 2003
Posts: 106
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23 Feb 2008 20:38 Re: How I can detect the 3 possible states in a bus RS485? |
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Hi FvM,
detect the 3 states will be posible because exist some transparent devices that implement 3 states without software. Please see:
http://www.ifs.com/downloads/datasheets_english/d1300.pdf
Thanks!
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FvM
Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany
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24 Feb 2008 12:42 Re: How I can detect the 3 possible states in a bus RS485? |
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Don't see any relation to the original question. If you connect a bridging device to a RS485 network, it has to act as any other network member does. It may either be able to understand the protocol or send, when the bus line is idle (and hopefully stays idle during send). But it has no means to detect, if all other drivers are three-stated or not.
P.S.: Generally, the operation of "dumb" (not knowing the protocol) bridging device is based on correct behaviour of the "intelligent" network members. The bridge simply duplicates all data to the other port, including possible collisions.
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kkdelabaca
Joined: 18 Apr 2003
Posts: 106
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24 Feb 2008 13:42 Re: How I can detect the 3 possible states in a bus RS485? |
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| Hi FvM, this is a transparent device that converts one RS485 signal in to Fiber optic system to extend the distance using fiber optic. You need a pair of these devices and somehow can detect the 3 states to send the status of the bus RS485 to the other transceiver.
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FvM
Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany
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24 Feb 2008 14:42 Re: How I can detect the 3 possible states in a bus RS485? |
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No, it's not necessary to detect the state of the bus for a transparent bridge. I have used this type of "transparent" bridges with PROFIBUS. They are simply duplicating data to the respective peer.
P.S.: The other point is: If a device would be able to detect tristate distinguished from idle, this wouldn't mean a thing. To send data safely, it's necessary to know, that no other device is expected to send shortly, this can't be seen from the momentary state.
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