Modbus Comms Highway

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betomachine Power-User Join Date: Apr 2007 Location: Austin, TX Posts: 367 Good Answers: 10	Modbus Comms Highway 10/16/2008 10:21 AM
	Chemical engineer trying to wire up an additional device into a Modbus network. I am running a DeltaV DCS v. 7.3 and have an existing serial port card that has an existing GC communicating by 485 to the card. The card is the Master while the GC is the slave. The facility is adding another GC and needs it to talk to the DCS via MODBUS. Assuming that I have enough licensing, I am planning on adding the new GC to the existing card on the same port as the existing GC. My question is how do I wire the MODBUS network to work? My current plan is to daisy-chain the new GC onto the end of the old GC and duplicate any terminations/end resistors needed on the new GC. Then I will call the new GC device 2 and set it up on the existing card. Does this sound workable? __________________ Money doesn't talk, it screams in your face.
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Anonymous Poster	#1 Re: Modbus Comms Highway 10/16/2008 11:23 PM
	For RS485 you need to wire the devices in parallel. That is make the same connections to each slave device. Usually the RS485 will be 2-wire so you need to connect A, B and GND to both of the GC's in parallel. Don't duplicate the termination resistor! Move it so it's at the end of the cable (the last GC). The idea of the resistor is to avoid reflections from the end of the cable, that's where it belongs. Some MODBUS slaves have the termination resistor internal to the unit, you may have to set a switch or jumper to connect it in. Hopefully you can copy the baudrate, parity and stop bits from the old GC onto the new GC. If not, for MODBUS normally 7 data bits, 1 stop bit and even parity are used. 9600 baud is common.
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betomachine Power-User Join Date: Apr 2007 Location: Austin, TX Posts: 367 Good Answers: 10	#2 In reply to #1 Re: Modbus Comms Highway 10/20/2008 9:50 AM
	Thank you for the advice, I will let you know how it turns out. __________________ Money doesn't talk, it screams in your face.
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