Tell me more ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.
up vote 2 down vote favorite

I've noticed my experiments in the lab with RS-485 work fine with fairly short cables, but termination resistors are needed for true installations. Is their presence or absence a function of the cable length, or other factors?

share|improve this question

2 Answers

up vote 8 down vote accepted

All RS-485 cables require termination. Some may just happen to work without them, but all should have them.

share|improve this answer
In point of fact, you'll probably find that they get left out just as often as they get put in. RS-485 is one of those standards that's often implemented by people who have no idea what they are doing, and is used frequently in a 'well it works, don't it?' kind of way. – Michael Kohne May 17 '12 at 23:04
up vote -3 down vote

Since terminating resistors load down the network, they should not be used unless they are required. Since reflected waves will dampen in 3-4 cycles, if the time for this to occur is less than one data bit width (or one half the bit width if sampling in the middle), the reflected waves will not interfere and terminating resistors are not required.

It is a simple enough calculation, figuring on the propagation velocity averaging around 65% of the speed of light: For a 9600 bps communication rate, on a 1000 foot cable, you have a round trip time of 3 usec, a dampening time between 9-12 usec, and a bit width of 10 msec. Therefore, each reflected wave will dampen out prior to you sampling each bit, so termination resistors are not required.

share|improve this answer
RS-485 is designed to support terminating resistors so loading the network down is not an issue. Leaving off terminating resistors could cause increased EMI and other issues and if the signal quality is bad enough it could cause errors even when the baud rate is slow. Note: I did not give this answer a -1, even though it probably deserves one. – David Kessner Aug 22 '12 at 19:39

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.