Understanding RS-485 Failsafe Biasing and Resistor Selection
The RS-485 standard defines a differential line driver and receiver system which provides better immunity to signal noise and ground shifts than similar single-line methods. These transceivers drive opposite polarity signals on the A and B lines which are interpreted by the differential receivers on the bus. In this architecture, Failsafe Biasing Networks hold the bus in a valid state when the bus is open, shorted, or idle.
The Importance of Failsafe Biasing
This is important because the RS-485 defines an indeterminate region between positive and negative input thresholds. Resistors for this network must be chosen so that they are strong enough to bias the bus out of this indeterminate range while still allowing transceivers to drive a valid LOW signal. The transceiver itself does not require specific failsafe resistor values; this is a system-level design consideration.
Internal vs. External Failsafe Implementation
Regarding specific hardware, SN65LBC184D has internal failsafe biasing which is capable of detecting open circuits and idle buses. The implementation is shown in the datasheet, where the A line has a 12-uA pull-up current source and the B line has a 12-uA pull-down current source. However, this implementation is not capable of detecting short-circuit conditions, though. Also note that in most cases internal failsafe is designed to apply to just that transceiver's node and wouldn't necessarily enable failsafe protection on every other node in the network like external failsafe biasing would.
If the transceiver did not built-in failsafe input threshold, the alternative way is to use additional resistors to create an external bias on the idle bus. You may want to look into a newer device like THVD1419. It improves upon the surge immunity of the older SN65LBC184 device and has an internal failsafe implementation capable of detected shorted-bus conditions as well.
Termination and Resistor Calculations
Typical termination values are selected to match the impedance of the cable being used. Choosing these values to match closely improves reflective dampening. When calculating the network, we use the worst case supply voltage Vs = 4.75V and noise level Vab= 0.25V. Note the minimum value for these resistors is dependent on the driver strength and their ability to overcome the external biasing to drive a valid LOW. In a system with less loading and more favorable line-matching, this value will extend to allow greater resistor values. For example, a system with two 150 Ohm termination will have a maximum allowed Rfs closer to 680 Ohms.
Key Design Parameters and Values
| Component / Parameter | Specified Value |
|---|---|
| Profibus cable Impedance | 150R |
| SN65HVD1176 Bias resistor | 390R |
| SN65HVD1176 Termination resistor | 220R |
| Worst case supply voltage (Vs) | 4.75V |
| Noise level (Vab) | 0.25V |