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"RS 422 multidrop comunication"

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nubarano - (11 posts) Mar-01-01, 11:43 AM (EST)

"RS 422 multidrop comunication"

I have to connect 20 devices (PH gauges)with RS-422 interface to a PLC. I would like to build a multidrop system to connect all the tools to the PLC in a net, but I know that the RS-422 standard does not have tri-state capability and it cannot work in a Multidrop system.
How I can build a net between all the PH gauges and PLC? Does a converter exist to pass from the RS 422 interface to the RS 485 interface and realize a Multidrop system?
Thanks for the answers

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RE: RS 422 multidrop comunication, Tom Jenkins, Mar-01-01, (1)
RE: RS 422 multidrop comunication, Jim Dungar, Mar-01-01, (2)

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Tom Jenkins - (768 posts) Mar-01-01, 01:21 PM (EST)

1. "RE: RS 422 multidrop comunication"

There is a lot of confusion (including some on my part) between RS422 and RS485. You may be able to use the RS422 devices on an RS485 multi-drop by jumpering the RTS/CTS lines.
Enrelec and Phoenix contact, among others, make adapters that you can use to bridge the two formats if needed.

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Jim Dungar - (158 posts) Mar-01-01, 06:07 PM (EST)

2. "RE: RS 422 multidrop comunication"

Conversion from RS422 to RS485 is easy, there are many manufacturers of off the shelf convertors. My favorite is B&B Electronics, they have a good application note on their website at www.bb-elec.com.
RS485 can be operated in a 2-wire mode where each device is "intelligent" so they are only connected during the transmission of data (similar to software handshaking). Typically your devices have to be built with this capability, although you can buy convertors that have "send data" control.
Another method of RS485 is 4-wire mode where one node is a "master" and all of the other nodes are slaves.
With either method a big question is: Do your gauges have individual addresses, so that only the correct one answers?
Good luck.

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nubarano - (11 posts)	Mar-01-01, 11:43 AM (EST)
"RS 422 multidrop comunication"
I have to connect 20 devices (PH gauges)with RS-422 interface to a PLC. I would like to build a multidrop system to connect all the tools to the PLC in a net, but I know that the RS-422 standard does not have tri-state capability and it cannot work in a Multidrop system. How I can build a net between all the PH gauges and PLC? Does a converter exist to pass from the RS 422 interface to the RS 485 interface and realize a Multidrop system? Thanks for the answers
	Top

Tom Jenkins - (768 posts)	Mar-01-01, 01:21 PM (EST)
1. "RE: RS 422 multidrop comunication"
There is a lot of confusion (including some on my part) between RS422 and RS485. You may be able to use the RS422 devices on an RS485 multi-drop by jumpering the RTS/CTS lines. Enrelec and Phoenix contact, among others, make adapters that you can use to bridge the two formats if needed.
	Top

Jim Dungar - (158 posts)	Mar-01-01, 06:07 PM (EST)
2. "RE: RS 422 multidrop comunication"
Conversion from RS422 to RS485 is easy, there are many manufacturers of off the shelf convertors. My favorite is B&B Electronics, they have a good application note on their website at www.bb-elec.com. RS485 can be operated in a 2-wire mode where each device is "intelligent" so they are only connected during the transmission of data (similar to software handshaking). Typically your devices have to be built with this capability, although you can buy convertors that have "send data" control. Another method of RS485 is 4-wire mode where one node is a "master" and all of the other nodes are slaves. With either method a big question is: Do your gauges have individual addresses, so that only the correct one answers? Good luck.
	Top

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