Baldor MIN1853 manual RS485 PC Connections, What is a termination resistor?

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

RX+

RX– LinStep+ TX+

TX-

DGND

Chassis

TX+

TX–

RX+ PC / Host Device

RX-

DGND

Figure 3-12 9 Pin RS-485 Cable Connections For CE Installations

RX+

RX–

LinStep+ TX+ TX-

DGND

Chassis

PEPE

TX+

TX–

RX+ PC / Host

RX- Device

DGND

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).

MN1853 Receiving & Installation 3-9