The NI 6583 utilizes the TI SN65LVDM180 LVDS receiver. This receiver has a built in fail-safe which returns all HIGH values when the line is disconnected (open circuit) or not being driven (high impedance). When this occurs, the LVDS receiver pulls each line of the signal pair to near VCC through 300kΩ resistors as shown in the figure below. The fail-safe feature uses an AND gate with input voltage thresholds at about 2.3 V to VCC – 0.4 V to detect this condition and force the output to a high-level, regardless of the differential input voltage.
The NI 6585(B) also utilizes the TI SN65LVDM180 receiver. However, there are external components in addition to the fail-safe circuit that can cause the lines to be in an indeterminate state when there is an open circuit (i.e. no connection made). Due to this, the fail-safe feature is not supported by this instrument.