How can I tell if I'll need termination on my RS485/RS422 connections?

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The purpose of termination is to match the impedance of a transmission line to the hardware impedance of the interface it is connected to. There is more than one way to add termination to an RS485/422 serial connection. The most commonly used is DC Termination, accomplished by attaching a resistor between the signal lines on the extreme ends of the transmission line.

For more information see these articles:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/763 - Search for "Termination Resistors"

https://en.wikipedia.org/wiki/RS-485 - Read the Overview. Click on the References and External links for more detailed supporting information.

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The rule of thumb for termination is:

• If the propagation delay of the data line is much less than one bit (pulse) width, termination is not needed.

This assumes reflections will damp out in several trips back and forth on the data line.

The calculations below show that termination is not normally required at slower serial speeds, e.g. 9600bps.

Here's how to calculate the propagation delay:

• Propagation delay = length of cable / propagation velocity

Propagation velocity is typically 66 to 75% of the speed of light (c).

Here is an example calculation for a 2000 ft data line at 9600bps with a propagation velocity of 0.66 c:

• 2000 ft of transmission line for a round trip of 4000 ft.
• c = 186,000 miles/s * 5280 ft in a mile or 982,080,000 ft/s.
• Propagation velocity = 0.66 * 982,080,000 or 648,172,800 ft/s.
• Propagation time = 4000 ft / 648,172,800 ft/s or about 6.2 µs.

If the reflections damp out in three round trips on the cable the signal will stabilize in about 18.6 µs. At 9600 bps one bit pulse is 104 µs wide.

The bit will be sampled at the center of the pulse (after 52 µs) so in this case termination is not required as the signal will have stabilized after about 20 µs.

So for cables 2000 ft or less termination is not needed at 9600bps.

Since termination adds to the complexity of the installation and can potentially load down RS485 devices, Lantronix recommends only using termination if it's required.

If termination is required a resistor value of 120Ω or greater should be used, and no more than 2 termination resistors should be used, one at each end of the RS485 transmission line.

Do not use termination resistors with a value of less than 90Ω.

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[Originally Published On: 06/12/2007 10:29 AM]