DPDT switch as data switch? - PLCS.net

Nearbyatom · January 26th, 2018, 11:27 AM

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?

gclshortt · January 26th, 2018, 12:04 PM

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,

g.mccormick · January 26th, 2018, 12:53 PM

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

James Mcquade · January 26th, 2018, 01:25 PM

it will work intermittently, if at all.

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

james

Nearbyatom · January 26th, 2018, 03:10 PM

Quote:

Originally Posted by gclshortt

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

Nearbyatom · January 26th, 2018, 03:12 PM

Quote:

Originally Posted by James Mcquade

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?

Mickey · January 26th, 2018, 03:33 PM

Quote:

Originally Posted by Nearbyatom

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.

DwSoFt · January 26th, 2018, 08:43 PM

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.

rguimond · January 28th, 2018, 06:24 PM

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

OkiePC · January 28th, 2018, 09:54 PM

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.

January 26th, 2018, 11:27 AM	#1
Nearbyatom Member Join Date: Jul 2017 Location: USA Posts: 38	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?

January 26th, 2018, 12:04 PM	#2
gclshortt Member Join Date: Dec 2014 Location: Cobourg Posts: 2,744	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, __________________ Garry https://www.accautomation.ca Connect with us on Facebook: facebook.com/accautomation/

January 26th, 2018, 08:43 PM	#8
DwSoFt Lifetime Supporting Member Join Date: Mar 2012 Location: Alberta Posts: 893	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. __________________ Dan Wiebe ----------------- Automation Manager Phoenix Energy Services

January 28th, 2018, 06:24 PM	#9
rguimond Lifetime Supporting Member Join Date: Jul 2009 Location: Escuminac Posts: 641	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 __________________ "If you can't measure it, you can't manage it!"

January 28th, 2018, 09:54 PM	#10
OkiePC Lifetime Supporting Member Join Date: Mar 2005 Location: ENE of Nowhere Oklahoma Posts: 10,068	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. __________________ It's not all the variables I am most concerned with, it's the undiscovered constants.



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January 26th, 2018, 12:53 PM	#3
g.mccormick Member Join Date: Jul 2012 Location: IN Posts: 548	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

January 26th, 2018, 01:25 PM	#4
James Mcquade Member Join Date: Oct 2007 Location: Tennessee Posts: 2,316	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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