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How to put bias resistors for PXI-8431/2 serial card

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Message 1 of 8
Please look at this installation guide.
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Message 2 of 8

Hi Mark,


I've already have this guide, and I am not sure where to put these two (pull-up & -down) 620 Ohm resistors on the PXI-8431/2 plug-in card's PCB itself.


I also have an ExpressCard version of the RS485 2-port module, and it comes with the (software) option of enabling the bias resistors (but this option seems not available for the PXI-8431/2).

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Message 3 of 8



The PXI-8431/2 should have factory installed 620 ohm bias resistors, however they are in sockets on the board, so that you can change these values if you would like to bias the network in more than one location. Multiple biasing can make a network more robust. In some cases, such as networks with long distances or weak device drivers, you may need to reduce these values in order for the device's drivers to overcome the biasing. Only the 2 port and 4 port versions have bias resistors built in.

The resistors are calculated to provide a 200 mV difference between the + and - inputs with a total termination of 50 ohms in place; 200 mV is the threshold for the receivers. This keeps the input in a known state when nothing is driving these lines.



You will need to add external line termination resistors:


Terminating resistors are used to prevent line reflections. In a multidrop network (a network with multiple devices) one resistor at each end of the network is all that is needed. The RS-485 specification calls for a 100-120 Ohm twisted pair cable. The terminating resistors should be chosen to match the impedance of the transmission line. Our in-line terminators are 120 ohm value (part# 182844-01), and can be purchased on-line.

None of our serial boards come with terminating resistors built-in. The terminating resistor must be placed on either side of the RS-485 network, and since the master 485 port could be in the middle of the bus, it does not make sense to put terminating resistors on these boards.


I hope this helps!


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Message 4 of 8
You may also find this an interesting read!
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Message 5 of 8

Hi Mark,


I already have termination resistors in my RS485 multi-drop network.


Following your message, I would consider the two RS485 adaptors (ExpressCard based and PXI- based) identical (i.e. both have bias resistors built-in, I can see 8x 620Ohm resistors on PXI-8431/2 serial card).


I have a VI code which runs okay with the ExpressCard RS485 adpator (mean voltage between A&B lines is ~0.4V when code running, 0.617 V code stopped).

However, the identical VI code won't run with the PXI-8431/2 adaptor (mean voltage between A&B lines is ~1.55V when code running, 2.13 V code stopped).


Any suggestions why the PXI-8431/2 won't work  ?


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Message 6 of 8



You may find it useful to know that PXI/PCI versions of the device draw their power from a 5 Volt bus and the ExpressCard/34 version draws its power from a 3.3 Volt bus, which could be causing the discrepancy.


Some things to try:


Do you have a duplicate PXI-8431/2 you could try and see if you get the same behaviour?

Have you updated to the latest drivers?



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Message 7 of 8



I have installed the driver NI-Serial v3.5 (that comes with the PXI-8431/2 serial card).


I have asked for a replacement of the card. Your colleague George dealt with the case a few days ago but I have not received any reply from him so far.

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Message 8 of 8