Instrument Control (GPIB, Serial, VISA, IVI)

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NI 9871 Bias Resistors

I've just been informed that the NI 9871 c Series Module doesn't have bias resistors fitted.


Has anyone taken one of these module apart before in order to fit them? It seems very strange to me that they aren't fitted internally.

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Message 1 of 17

Hello Richard,


I have contacted our Product Support Engineer for the specific module to enquire why these are not fitted internally. Although a bias value of 620 ohms is common for the bus, we found that in many circumstances this is not always the case and customers wanted the flexibility to specify these themselves.


If you feel opposed to the above, certainly let us know and we can consider this feedback when making future design decisions.


Are you still having difficultly with fitting these manually and is there any further support I can provide? 



Jamie Jones
Applications Engineer
National Instruments
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Message 2 of 17

I'm still doing some investagitive work on this, currently as soon as I put any line termination resistors in the communication drops out. I'm trying to find out if it's the cSeries module or the device itself.


I'm not sure it's possible to put the biasing resistors in as there's no access to the +ve supply and the drivers as that's internal to the module.

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Message 3 of 17

OK, here's some information I've found.


Attached is an image of letter a sent over RS485 from another device.


Here's a youtube video of the NI 9871 sending a letter a.


You can see there an erroneous high signal sent at the start of the transmission which varies in length, and that the stop bits aren't as long as they are supposed to be.


Can anyone offer an explanation of this?

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Message 4 of 17

Hello Richard,


Which resistor values are you currently using and how is this implemented?


Do you know the characteristic impedance of the transmission line you are terminating?


Kind Regards,

Jamie Jones
Applications Engineer
National Instruments
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Message 6 of 17



I'm using 120 Ohm termination resistors and about 70m of the following cable: Belden 3106A 010500.



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Message 7 of 17



I would be better to use a failsafe terminator at one side and a passive at the other side. But you need a connection to the power at either side to connect the failsafe terminator.


Also you can try to add a 0V connection between the devices. I know that many will say that this is not necessary, but if the difference between 0V levels are outside the common-mode range of the RS-485 transceivers the 0V connection might help.



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Message 8 of 17



I have the "Ground" wire connected between all of the devices as well as the Tx/Rx+ and Tx/Rx- wires.


I believe the 9871 has a connection between its ground pin and the 0v of the isolated 24v supply.


Is the failsafe terminator that you are talking about the 560 Ohm betwen the 5v and Tx/Rx+ and the 560Ohm between the 0v and the Tx/Rx-?

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Message 9 of 17

Hi Richard,


We use 390 ohm to Vcc and 0V and 150 Ohm in between these resistors.

But there are other values possible. It is not that critical. Not at that distance and speed.


Can you something about the device you connect. Is it working OK with another interface ?


I had a better look at the scope video. Looks to me that the Tx enable of the RS-485 transceiver is always starting at the same moment and stops too soon. From the timing I assume that the baudrate is 9600 baud.




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Message 10 of 17