GE Industrial Solutions EntelliPro ES - Manual (Page 142)

EP OS MOTOR MANAGEMENT SY STEM – INSTRUCTION MANUAL 4.5
CHAPTER 4: COMMUNICATION
4.1.4 Modbus topology
Figure 4-2 illustrates the standard two-wire Modbus topology. To minimize the effects of reflections from the ends of the RS-
485 cable caused by impedance discontinuities in the system, a line-terminating resistor on the RC network must be placed
near each end of the bus, as illustrated in Figure 4-2.
If one or more devices require polarization, one pair of resistors must be attached to the RS-485 balanced wire pair, as follows:
 Pull-up resistor to 5 V on the positive line
 Pull-down resistor to common on the negative line
These resistors must be between 450 and 650 Ω. The latter may allow a higher number of devices to be connected to the
serial bus.
NOTE: Pull-up and pull-down resistors are recommended in an industrial environment .
HOST (PCL/PC)
M M C
o o o
d d m
b b m
u u o
s s n
5V+ Common
– +
TERMINATING RESISTOR
OR RC NETWORK
Common
TERMINATING RESISTOR
OR RC NETWORK
PE PE
EntelliPro ES N EntelliPro ES N
PE PE
Figure 4-2: Two-wire Modbus topology.