Data Acquisition

terminating resistors, approximately two times the characteristic impedance (Z o ) of the line,are fitted at the extreme ends. These resistors are typically between 100 and 500 Ω andreduce the effect of reflections at the ends of the line. For twisted pairs, the characteristicimpedance is typically between 100 to 120 Ω.During operation, there maybe periods of time when all RS-485 line drivers are off and thecommunications lines are in ‘idle’. In this condition, the lines are susceptible to noise pickup,which can be interpreted as random characters on the communications line. A solution to thisproblem is to incorporate 1 kΩ bias resistors as indicated in the terminated multidrop networkof Figure 6.8. These resistors will maintain the data lines in a mark condition when thesystem is ‘idle’.Figure 6.8RS-485 multidrop networks6.3.1 RS-485 repeatersRS-485 line drivers are designed to drive up to 32 receivers on a network. This limitation canbe overcome by employing an RS-485 repeater. The repeater is a two-port device that retransmitsdata received on one side, at full voltage levels, to the network on the other side.Consequently, another 31 devices may be connected for each repeater used, as shown inFigure 6.9.