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convert rs232 to rs485

Is it possible on a compact Fieldpoint to use one of the rs232 ports and convert it to an rs485 port (preferably to a multi-drop).  I've found several converter devices on the market for rs232 to rs485 conversion.  So I gues the question is, can Labview / cFP pass multi-drop info through the rs232 port to be converted to rs485?
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Message 1 of 13
Yes it can.  The handling of multidrop is all in the addressing scheme and depends on the software you write and what communication protocols you use (such as modbus).  I am using an RS-232/RS-485 converter to talk to a single device, but other devices could be added as well if I needed to.  I've used several RS-232/485 converters from B&B electronics  It is just a matter of setting some switches to set it up for 2-wire or 4-wire or RS-422 or RS-482.
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Message 2 of 13
Thanks, Ravens Fan for your response.  I'm pretty new to the serial communication world, so bear with me if I ask some stupid questions.  I am going to be using Modbus, So is there a difference in the data packet when sending via 232 vs 485?  i.e.  is compact Fieldpoint and/or Labview going to have a problem if I try to send Modbus data that is addressed for 485 devices through the 232 port (converted using the 232/485 converter)? 
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Message 3 of 13
Neither the fieldpoint nor Labview will care which kind of port it gets sent out.
RS-232 and RS-485 just defines how the data is transmitted on a wire.
In RS-232 you have 3 wires, a transmit, a receive, and a ground.  A bit 1 or a bit 0 is just either a high voltage or low voltage (I don't remember which was 1 and which was 0) on a wire relative to the ground.  There is one wire for each direction.  Transmit on one device connects to the receive on the other, and vice versa.  As a result, it is only point to point.
In RS-485 4-wire, you have a pair wires for transmit and a pair for receive.  The signal is based on the differential voltage on the pair of wires.  There is usually a 5th wire for ground reference, but that may not be needed.  This can be multi-drop.  You would have a master device whose transmit wires go to the receive of all the other devices, and the receive comes from the transmit of all the other devices.  Transmitting and receiving can occur simultaneously like on an RS-232 setup.
In RS-485 2-wire, you have only a single pair of wires which are transmit and receive.  Again the signal is based on the differential voltage.  (There may be third wire for ground as well.)  All devices share this pair of wires.  Only one device can transmit at a time.
There is also the designation of RS-422.  I am not sure how this is technically any different from RS-485 4-wire.
That all defines the way data is sent on the wires.  The format of the data itself is whatever it needs to be to make the communication work and doesn't care how it is transmitted on the wires.  The RS-232/485 converter takes care of the electrical state of the wires and does the conversion from a single transmit and receive wire relative to a common ground on the 232 side to a single pair (or 2 pairs of wires) on the 485 side.  The converter handles this automatically and the behavior is usually transparent.  Labview will sent out data formatted to the Modbus protocol on either an RS-232 or RS-485 port without any problems.
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Message 4 of 13

I've seen 232 to 485 converters listed as "isolated" and "non-isolated" what's the difference, and how do I know which one I need?


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Message 5 of 13

An isolated converter will completely detach the wiring from one side of the converter to the other.  They will use optoisolators or sometimes transformers to get the signals across.  This is so if there is a high voltage fault on one side that causes a high voltage to land on the signal wires, the high voltage won't get passed through.  It is for safety and to protect the electronics on the other side.  Of course an isolated version costs more, but if your dealing with high voltage electronics such as a 480 VAC motor controller, it is probably a good idea to get it. 

A non-isolated converter will pass through some of the wires such as the ground wire, or +5VDC power wires.  If your dealing with lower voltage electronics where there isn't a worry about safety or a need to protect the electronics from electrical faults, the non-isolated converter should work fine.

Message Edited by Ravens Fan on 07-26-2007 11:44 AM

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Message 6 of 13
Great, yes they would be low voltage particle counters and fan controllers.
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Message 7 of 13
you can use the Hexin HXSP-2108C Industrial Level Optical Isolation RS-232 To RS-485/RS422 Converter, the Interface equipment avaible:128 RS-485/RS-422 interface equipment , it also support optical isolation,lightningproof and surge protection .
rs232-rs485/rs422 converter,usb to serial,serial server
keeping communicate fast and secure
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Message 8 of 13

thanks,I will study it.



pc adapter world:

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Message 9 of 13

Yes, this is definately possible, you can use a standard RS232 to RS485 converter to convert the RS232 to a multi-drop RS485, I did that sucessfully with several devices through Labview.

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Message 10 of 13