TE Technology TC-24-25 - Manual (Page 16)

16
APPENDIX A RS-485 Cable Termination Requirements
A twisted pair of wires for RS-485 applications may connect up to 32 ports for half-duplex data
transmission. An optional shield around the twisted pair helps reduce unwanted noise, and is connected
to ground at one end.
Proper termination of the cable is very important. If the cable is not correctly terminated with its
characteristic impedance, normally 120 ohms, distorted waveforms will result. Unwanted reflections
signals may occur which can distort or interrupt data transmission.
Cable terminating resistors are required at each physical end of the cables to prevent theses unwanted
reflections (Refer to Appendix B Multiple Controller Wiring Diagram). The typical differential output is
2 volts with two 120 ohm terminating resistors, causing 33mA of DC current in the cable when no data is
being sent. The preferred terminating resistor type is a metal film rated at ¼ watt.
This termination method permits connections up to 4,000 feet of RS-485 cable.
NOTE:
1) If an RS-232 to RS-485 converter is being used (other than the one supplied with the TC-24-25), please
check the converter manual to determine if a 120 ohm terminating resistor is included. The converter
supplied with the TC-24-25 has a 120 ohm resistor already installed within the converter. Also, each TC-
24-25 includes a 120 ohm terminating resistor. In multiple controller configurations, the resistor should
be removed from each controller except for the last controller in the line. Again, refer to Appendix B
Multiple Controller Wiring Diagram.
2) However, a string of TC-24-25 controllers may cause low signal levels from the RS-485 driver circuit
depending on the driver circuit being used, line length, wire diameter, number of controllers, etc. In this
case you may want to use a larger value (higher resistance) terminating resistor or remove it completely if
you are experiencing communication problems.