Reason for 100 Ohm Resistor on Transmitter of the NI 6585

Updated Jun 15, 2018

Reported In

Hardware

  • NI-6585

Issue Details

I noticed on the DIO Data Channel block diagram of the NI 6585 specifications (see Figure 1) that there is a 100 Ohm resistor on the transmitter side of the NI 6585.  Low-voltage differential signaling (LVDS) requires 100 Ohm termination on the receiver side, but not on the transmitter side.  Wouldn't adding the 100 Ohm resistors to both the transmitter and receiver halve the available current and therefore not meet the specifications of LVDS?

Solution

LVDS is a bidirectional standard that requires a 100 Ohm resistor on the receiver end of the LVDS circuit. Therefore, if a device wants to transmit to the NI 6585, a 100 Ohm resistor is required for termination in the NI 6585.  This does drop the the load impedance to 50 Ohms which halves the available current.  

The NI 6585 overcomes this decrease in load impedance by using a low voltage differential multipoint (LVDM) buffer rather than an LVDS buffer to output the current.  LVDM allows for double the current that LVDS provides which makes up for the halved load impedance.

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