The internal fail-safe circuitry was designed such that the input common-mode (V_IC) and differential (V_ID) voltages must be observed. To ensure the outputs of unused or inactive receivers remain in a high state when the inputs are open-circuited, shorted, or terminated, extra precaution must be taken on the active signal. In applications where the drivers are placed in a high-impedance mode or are powered-down, TI recommends that for 1, 2, or 3 active receiver inputs, the low-level input voltage (V_IL) must be greater than 0.4 V. As in all data transmission applications, it is necessary to provide a return ground path between the two remote grounds (driver and receiver ground references) to avoid ground differences. Table 8-2 and Figure 8-3 through Figure 8-5 are examples of active input voltages with their respective waveforms and the effect each have on unused or inactive outputs. Note that the active receivers behave as expected, regardless of the input levels.

Figure 8-3 Waveform 1

Figure 8-4 Waveform 2

Figure 8-5 Waveform 3

Figure 8-6 Waveform 4

In most applications, having a common-mode input close to ground and a differential voltage larger than 2 V is not customary. Because the common-mode input voltage is typically around 1.5 V, a 2-V V_ID would result in a V_IL of 0.5 V, thus satisfying the recommended V_IL level of greater than 0.4 V.

Figure 8-7 plots seven different input threshold curves from a variety of production lots and shows how the fail-safe circuitry behaves with the input common-mode voltage levels. These input threshold curves are representative samples of production devices. The curves specifically illustrate a typical range of input threshold variation. The AM26LV32 is specified with ±200 mV of input sensitivity to account for the variance in input threshold. Each data point represents the input’s ability to produce a known state at the output for a given V_IC and V_ID. Applying a differential voltage at or above a certain point on a curve would produce a known state at the output. Applying a differential voltage less than a certain point on a curve would activate the fail-safe circuit and the output would be in a high state. For example, inspecting the top input threshold curve reveals that for a V_IC that is approximately 1.6 V, V_ID yields around 87 mV. Applying 90 mV of differential voltage to this particular production lot generates a known receiver output voltage. Applying a V_ID of 80 mV activates the input fail-safe circuitry and the receiver output is placed in the high state. Texas Instruments specifies the input threshold at ±200 mV, because normal process variations affect this parameter. Note that at common-mode input voltages around 0.2 V, the input differential voltages are low compared to their respective data points. This phenomenon points to the fact that the inputs are very sensitive to small differential voltages around 0.2 V V_IC. TI recommends that V_IC levels be kept greater than 0.5 V to avoid this increased sensitivity at V_IC ≈ 0.2 V. In most applications, because V_IC typically is 1.5 V, the fail-safe circuitry functions properly to provide a high state at the receiver output.

Figure 8-7 V_IC vs V_ID Receiver Sensitivity Levels

Figure 8-8 represents a typical application where two receivers are not used. In this case, there is no need to worry about the output voltages of the unused receivers because these are not connected in the system architecture.

Figure 8-8 Typical Application With Unused Receivers

Figure 8-9 shows a common application where one or more drivers are either disabled or powered down. To ensure the inactive receiver outputs are in a high state, the active receiver inputs must have V_IL > 0.4 V and
V_IC > 0.5 V.

Figure 8-9 Typical Application Where Two or More Drivers Are Disabled

Figure 8-10 is an alternative application design to replace the application in Figure 8-9. This design uses two AM26LV32 devices instead of one. However, this design does not require the input levels be monitored to ensure the outputs are in the correct state, only that they comply to the RS-232 standard.

Figure 8-10 Alternative Solution for Figure 8-9

Figure 8-11 and Figure 8-12 show typical applications where a disconnected cable occurs. Figure 8-11 illustrates a typical application where a cable is disconnected. Similar to Figure 8-9, the active input levels must be monitored to make sure the inactive receiver outputs are in a high state. An alternative solution is shown in Figure 8-12.

Figure 8-11 Typical Application Where Two or More Drivers Are Disconnected

Figure 8-12 is an alternative solution so the receiver inputs do not have to be monitored. This solution also requires the use of two AM26LV32 devices instead of one.

Figure 8-12 Alternative Solution to Figure 8-11

When designing a system using the AM26LV32, the device provides a robust solution where fail-safe and fault conditions are of concern. The RS422-like inputs accept common-mode input levels from −0.3 V to 5.5 V with a specified sensitivity of ±200mV. As previously shown, take care with active input levels because this can affect the outputs of unused or inactive bits. However, most applications meet or exceed the requirements to allow the device to perform properly.