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Frequently Asked Questions
GENERAL QUESTIONS
6) WHEN DO I NEED TO USE 120 OHM TERMINATION?
Answer: RS422 and RS485 devices are typically connected using 120 Ohm impedance cable. A signal traveling along a transmission line (in our case, an RS422 or RS485 cable) will reflect off the end of the cable and back in the opposite direction if the signal encounters a change in transmission line impedance. A worst case scenario is an open cable. With an open cable, the signal will experience a change in impedance of 120 Ohm (the cable) to infinity (the open cable connection). The resulting signal reflection results in a distorted, ringing pulse traveling back up the cable. The distorted signal reflection is experienced as noise and interference on the cable. Termination resistors are connected at the ends of the cable to reduce these transmission line signal reflections. However, since the termination resistors increase the load on the data circuit, we recommend using termination resistors only when the data rate is over 19,200bps, or if the cable distance exceeds 660 feet (200m). Note that termination resistors are used only on the devices at the far ends of the network. In addition, any stub cables used along the transmission line to connect to RS422 or RS485 devices should be kept as short as possible. Many of our converters come with 120 Ohm termination resistors jumpered onto the terminal blocks.
