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Old January 26th, 2018, 11:27 AM   #1
Nearbyatom
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DPDT switch as data switch?

I have a RS485 modbus network. I'm looking at an HMI and a modem as a masters, and I have a proprietary board as a slave.

Obviously we can't have 2 masters on the same network. So I was thinking of using a DPDT switch to switch between masters. RS485 is 2 wires, so have NO wired to 1 master, and the NC wired to the 2nd?

Thoughts?
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Old January 26th, 2018, 12:04 PM   #2
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Theoretically this it is possible. The system is not usually designed for two masters unless using Modbus TCP (Ethernet). You will have to stop the master that is not communicating to the slave. This may be hard to do.
Can the HMI or Modem be a slave as well?
Can your proprietary board have another Modbus communication port? (Set up another network)

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Old January 26th, 2018, 12:53 PM   #3
g.mccormick
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I've done this before, it worked intermittently until the relay got a little oxidation. I went to a port switcher:

https://www.blackbox.com/en-us/store...ontact/SW1047A
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Old January 26th, 2018, 01:25 PM   #4
James Mcquade
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it will work intermittently, if at all.

the switch contacts will bounce (chatter) as they close and will create issues.

james
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Old January 26th, 2018, 03:10 PM   #5
Nearbyatom
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Quote:
Originally Posted by gclshortt View Post
Theoretically this it is possible. The system is not usually designed for two masters unless using Modbus TCP (Ethernet). You will have to stop the master that is not communicating to the slave. This may be hard to do.
Can the HMI or Modem be a slave as well?
Can your proprietary board have another Modbus communication port? (Set up another network)

Regards,
I don't think the HMI or modem can be a slave. By definition they are polling for information. The proprietary board doesn't have another com port
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Old January 26th, 2018, 03:12 PM   #6
Nearbyatom
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Quote:
Originally Posted by James Mcquade View Post
it will work intermittently, if at all.

the switch contacts will bounce (chatter) as they close and will create issues.

james
If the relay was SSR, I shouldn't get bounce/chatter would I?
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Old January 26th, 2018, 03:33 PM   #7
Mickey
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Quote:
Originally Posted by Nearbyatom View Post
If the relay was SSR, I shouldn't get bounce/chatter would I?

You could try it, but leakage may be an issue with solid state.

Let us know if you do.
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Old January 26th, 2018, 08:43 PM   #8
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In this situation I typically use an arbitrator (google scadalink ip100 for example) or an intermediate device (such as a scadapack) that polls the device and then the hosts poll it.
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Old January 28th, 2018, 06:24 PM   #9
rguimond
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I've had success using DTDT relays to do exactly what you're wanting to do. I had only several devices that could only be on RS-232. Depending on how fast you need the data, it should work just fine
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Old January 28th, 2018, 09:54 PM   #10
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Use bifurcated gold plated contacts and it will probably work well. If you switch the relay mid-message you might get an error, but most devices will handle that fine and resume normal functioning.
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