Fail-safe biasing

AN1702

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Fail-safe biasing

In an RS-485 network, a driver is always active on the line. Therefore, each receiver has

differential input voltages, which exceed the 200 mV minimum threshold voltage, ensuring a

known logic state at its output. When a receiver is disconnected from the bus (receiver

inputs floating), its output assumes an undetermined logic state. The ST485EB has an

internal fail-safe feature, which avoids this problem. A weak internal biasing ensures a

receiver differential voltage higher than 200 mV, and therefore a valid logic state at its output

under floating inputs conditions.

A different situation occurs when there are no active drivers on the network (idle-bus). In this

case, the receivers are connected to the bus and to the termination resistors. The

termination resistors decrease the differential voltage near 0 V, and the internal fail-safe is

not able to ensure a proper voltage level.

Some transmission protocol (for example the UART protocol) requires a high logic state of

the bus when in idle state. In order to solve this problem, two biasing resistors should be

applied to the two line wires, giving a proper biasing of the line (see also the application note

"fail-safe biasing for ST485"). In a terminated bus with a 120

Ω termination resistor at each

end, the equivalent resistance is 60

Ω (120 Ω/120 Ω). In order to produce a differential

voltage at its leads higher than 200 mV, a current of 4 mA should flow into it. In order to

guarantee this current without unmatching the characteristic impedance of the line,

500/600

Ω resistors should be used (see Figure 5).

Figure 5.

External fail-safe and line DC termination

The external fail-safe circuitry should be placed at one end of the bus or at each end of it,

using, in this case, twice the value calculated for one biasing circuit (1 K/1.2 K

Ω) and, of

course, it should not be disconnected for any reason.

The external fail-safe circuitry draws current; therefore it loads the driver outputs. This extra

load should be taken in account defining the maximum number of transceivers allowed on

the bus. The equivalent load of the fail-safe circuitry, seen by the active driver is, for

Ra = Rc = 500

Ω, 24UL. Since the maximum allowed on the bus is 32UL, the available unit

loads should be 32UL - 24UL = 8UL. Using the ST485EB (1/8UL per transceiver) the

maximum number of transceivers should be: 8UL / (1/8UL per transceiver) = 64

transceivers.