Instrument Control (GPIB, Serial, VISA, IVI)

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Biasing resistors on PCI RS485 / 8 port

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Is there a difference between the PCI 8433/4 and PCI 8431/8 in terms of isolation, bias resistors?  Are the bias resistors added on the 8 port card?  If not, what is the reason for leaving them out?  If I need to add the biasing and terminating resistors, is there a board layout that shows which pads are for the resistors?



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Message 1 of 9
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The PCI-8433/4 has 2000 V digital, port to port isolation and comes with 620 ohm built-in bias resistors.  The PCI-8431/8 has no isolation and does not come with bias resistors, and there are no pads on the PCB for them.  Due to the nature of the 68-pin connection to the 8 DB 9 ports, bias resistors will need to be added externally.
Robert Mortensen
Software Engineer
National Instruments
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Message 2 of 9
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I see pads, on the RS485/8 port card.  I've ohmed them out to verify their position with respect to Tx, Rx, VCC, and GND.  Do you see any problems adding the biasing and terminating resistors on the card? 
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You're right, the pads are there.  I verifed this after posting to you.  However, soldering in your own resistors voids the warranty on the board, so it's up to you.
Robert Mortensen
Software Engineer
National Instruments
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Hi Robert;

I'm in same trouble .I have a PCIe 8431/8 card and I want to put bias resistor on it .But I could not understand on which pad do I have to use for soldering the resistor.Could you please help me about the pad ID of bias resistors?I think the marking on PCB "UL94V-0  198108B-02 "gives you idea about the version of the PCB. 

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I have a Serial/RS-232/RS-485 Isolated , It seems that National Instruments never release a manual or a simple PDF for  resistors location on these boards.

Strange,  a company that produce tons of documentation actually do not know the way the 16 pads for resistors are to be set in that board.

I have ask to NI Italy support and they where not able to answer, just the suggestion to look at the very  generical document on Seriallike this one http://www.ni.com/pdf/manuals/373096a.pdf.

I had place a similar request to NI support  in 2007 and had the same answer: nothing !

Next time before buying a NI product I will think twice....

 

Anyway i suppose looking at the board that locations XR1,XR2,XR3,XR4 are for termination of each port so the other resistors pads should be  for bias.

 

 

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I was wrong the resistors pads on the board are ONLY for the bias, put 620Ohm resistors everywhere and it should be ok,The termination resistors must be installed externally the board . This was the information I have received from italian NI support

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Message 7 of 9
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Hi lucamax62

 

 

May I ask how you finally set the bias resistor on the 485 bus?

looking forward to your reply

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Message 8 of 9
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Hi,

 

You are replying to a almost 10 years old thread. !!

 

A RS-485 line should be terminated at both end with a resistor of 120 Ohm (or similar value)

 

Kees 

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