JAJSGA9C September 2018 – September 2019 ISO1500
The differential receiver of the ISO1500 device has failsafe protection from invalid bus states caused by:
The differential input of the RS-485 receiver is 0 in any of these conditions for a terminated transmission line. The receiver outputs a failsafe logic-high state so that the output of the receiver is not indeterminate.
The receiver thresholds are offset in the receiver failsafe protection so that the indeterminate range of the input does not include a 0 V differential. The receiver output must generate a logic high when the differential input (VID) is greater than 200 mV to comply with the RS-485 standard. The receiver output must also generate a output a logic low when VID is less than –200 mV to comply with the RS-485 standard. The receiver parameters that determine the failsafe performance are VTH+, VTH–, and VHYS. Differential signals less than –200 mV always cause a low receiver output as shown in the Electrical Characteristics table. Differential signals greater than 200 mV always cause a high receiver output. A differential input signal that is near zero is still greater than the VTH+ threshold which makes the receiver output logic high. The receiver output goes to a low state only when the differential input decreases by VHYS to less than VTH+.
The internal failsafe biasing feature removes the need for the two external resistors that are typically required with traditional isolated RS-485 transceivers as shown in Figure 29.