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Receiving & Installation 3-9MN1853
RS485 PC Connections
Standard RS485 connections are shown in Figure 3-11 and 3-12. Maximum cable
length is 3280 ft (1000M).
Figure 3-11 9 Pin RS-485 Cable Connections For UL Installations
TX-
DGND
TX+
RX+
RX–
RX-
DGND
RX+
TX+
TX–
Chassis
LinStep+
PC / Host
Device
Figure 3-12 9 Pin RS-485 Cable Connections For CE Installations
TX-
DGND
TX+
RX+
RX–
RX-
DGND
RX+
TX+
TX–
Chassis
PE PE
LinStep+
PC / Host
Device
Note: For CE installations, connect the overall shield at each end of the cable to PE. The
voltage potential between the PE points at each end of the cable must be Zero Volts.
RS485 Multi-Drop Connections
What does termination or a termination resistor do?
Termination resistance is used to match the impedance of the load to the
impedance of the transmission line (cable) being used. Unmatched impedance
causes the transmitted signal to not be fully absorbed by the load. This causes a
portion of the signal to be reflected back into the transmission line (noise). If the
Source impedance, Transmission Line impedance, and Load impedance are all
equal, these reflections (noise) are eliminated.
Termination does increase load current and sometimes changes the bias
requirements and increases the complexity of the system.
What is a termination resistor?
A resistor is added in parallel with the receiver input to match the impedance of the
cable being used. Typically, the resistor value that is used is 100 ohm or 120 ohm.
Resistors with 90 ohms or less should never be used.
Where are these resistors placed?
Terminators or Termination resistors are placed in parallel with the receiver at both
ends of a transmission line. This means that you should never have more than
two terminators in the system (unless repeaters are being used).
How many resistors should my system have?
Terminators or Termination resistors are placed in parallel with the receiver at both
ends of a transmission line. This means that you should never have more than
two terminators in the system (unless repeaters are being used).