Fail-safe in multipoint transmission buses

AN1690

10/15

7

Fail-safe in multipoint transmission buses

As discussed in the example of the calculation for fail-safe resistors, when calculating their

values, the following conditions must be satisfied:

●

The driver must be able to develop a differential output voltage Vod >=1.5 V

The excessively low resistance of the pull resistors could affect the driver differential output

voltage. In a multipoint application, where up to 32 transceivers could be connected in

parallel to the transmission line (Figure 9), the differential output voltage drops, due to the

equivalent input impedance of all the receivers connected. A minimum input impedance of

12 k

Ω for each receiver is required, so in the worst case of a fully loaded network (32 unit

loads) the equivalent resistance seen by the active driver is (12 k

Ω / 32) = 375 Ω.

Figure 9.

Multipoint transmission line with ST485EB

This value should be reduced in order to take into account that there are 31 drivers in a high

impedance state, each with a leakage current. However in the ST485EB device this current

is less than 10 µA, so its effect can be neglected. With regard to the ground shift, the

previous schematic can be modelled as shown in Figure 10, in order to verify the driver

output voltage capability.