RS-485 bus termination

A termination resistor is required on devices located at the ends of the RS-485 transmission line. Terminating the cable end preserves data integrity by preventing signal reflections. If there is only one device connected to the Remote port, it is considered the last device and should be terminated. For a one-way bus installation terminate the last device on the bus. In a split bus configuration, terminate the devices installed on each end of the two buses. More than two lines out of the master clock or time server are not allowed. Never branch off in the middle of the line from one device to two other devices! This can cause reflections in the bus. Some input devices include a built-in termination switch to terminate the RS-485 bus when required. The TimeView display clocks and model 8185 TimeBurst have rear panel DIP switches to terminate the clock if it is the last device on the line. The Model 8175 and 8177 termination switch is DIP switch number 10. The Model 8185 termination switch is DIP switch number 1. The Model 9175 display clock, Models 8188 and 9188 ethernet time server and Model 8179T TimeTap do not have a built-in termination resistor. If one of these devices is at the end of the line, install the supplied 120 ohm resistor across the + Data and -Data input pins as shown in figure 1. This will terminate the end of the line.

Table: Termination requirements by product
Product	Model	End of Bus Termination
TimeView TV230/400	8175/8177	DIP switch 10
TimeView TV230/400	9175	120 Ohm resistor
TimeBurst	8185	DIP switch 1
Ethernet Time Server	8188/9x88	120 Ohm resistor
TimeTap	8179T	120 Ohm resistor

FIGURE 1: Resistor termination

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