Bronkhorst Modbus slave interface - Manual (Page 11)

BRONKHORST ®
2.4 Termination
For best quality of data transfer Modbus should be terminated correctly.
2.4.1 Termination resistors
A resistor is added in parallel with the receiver’s “A” and “B” lines in order to match the data line characteristic
impedance specified by the cable manufacturer (120 Ω is a common value). This value describes the intrinsic
impedance of the transmission line and is not a function of the line length. A terminating resistor of less than 90 Ω
should not be used. Termination resistors should be placed only at the extreme ends of the data line (see Termination
schematics resistors RT1 and RT2), and no more than two terminations should be placed in any system that does not
use repeaters.
2.4.2 Biasing resistors
When an RS-485 network is in an idle state, all nodes are in listen (receive) mode. Under this condition there are no
active drivers on the network. All drivers are tri-stated. Without anything driving the network, the state of the line is
unknown. If the voltage level at the receiver’s A and B inputs is less than ±200 mV the logic level at the output of the
receivers will be the value of the last bit received. In order to maintain the proper idle voltage state, bias resistors
must be applied to force the data lines to the idle condition. Bias resistors are nothing more than a pull-up resistor
(RB1) on the data D1 Modbus (B/B') line and a pull-down (to ground) on the data D0 Modbus (A/A') line. The
“Termination schematic” illustrates the placement of bias resistors on a transceiver. The value of the bias resistors is
dependent on termination and number of nodes in the system. The goal is to generate enough DC bias current in the
network to maintain a minimum of 200 mV between the B and A data line. Consider the following example of bias
resistor calculation.
Ideal situation:
Termination resistors: 120 Ohm
Receiver resistance: omitted
Bias supply voltage: 5Vdc
Wanted situation is a minimum of 200mV between A and B lines and a common mode voltage of 2.5V.
Minimum current therefore: 200mV / 60 Ohm = 3.33mA
Total maximum bias resistor value is (5V – 0.2V)/3.33mA = 1440 Ohm.
The maximum value of each biasing resistor: 720 Ohm.
Situation with 127 nodes:
Termination resistors: 120 Ohm
Receiver resistance: 12 KOhm
Number of instruments: 127
Bias supply voltage: 5Vdc
Wanted situation is a minimum of 200mV between A and B lines and a common mode voltage of 2.5V.
Total termination resistance: 120 // 120 // 12000* 127 = 120 // 120 // 94.5 = 36.7 Ohm
Minimum current therefore: 200mV / 36.7 Ohm = 5.45mA
Total maximum bias resistor value is (5V – 0.2V)/5.45mA = 880 Ohm.
The maximum value of each biasing resistors: 440 Ohm.
Lower values may be used. (Depending on maximum power consumption of the resistors)
Bronkhorst advices the following resistor values for the following voltages.
Supply voltage
termination
Termination
resistors
Bias Pull-up
resistor
Bias Pull-down
resistor
+5V 121 Ohm 392 Ohm 392 Ohm
+10V 121 Ohm 1210 Ohm 392 Ohm
+15V 121 Ohm 2210 Ohm 392 Ohm
+24V 121 Ohm 3480 Ohm 392 Ohm
Page 11 Modbus interface 9.17.035