HSEROUT/HSERIN with RS422/485 Driver

[Darrel Taylor]

Right, I love that diode arrangement.

It reduces the current dramatically since you don't need that 100Ω resistor.
The "Fail-Safe resistors can be much higher values too, I used 2K on mine.

And it allows a "Star" network where you can connect devices anywhere on the buss, instead of a long multi-drop network with a terminating resistor at the end.
And the more devices you connect, the better the signal gets.

Can't imagine why anyone would terminate RS485 any other way.

[Andy Wood]

Hi Darrel,

I must admit I have only quickly glanced over the article. One thing that puzzles me a bit is how the diode arrangement actually works to dampen the reflections on the buss.

I have a bit of a background in RF and I always assumed the terminating resistor was selected to be approx the same impedance as the line. This is transmission line theory and, in this instance, the termination resistance equals the characteristic impedance of the transmission line, and therefore the line operates with low reflections or SWR (Standing Wave Ratio). I would like to understand more on how the diodes produce the same effect without the terminating resistor. However it works it sure seems to be better arrangement.

Andy

[Darrel Taylor]

Well, here's a quote from the previously mentioned appnote... http://www.maxim-ic.com/appnotes.cfm/an_pk/1090

Schottky-Diode Termination
Schottky diodes offer an alternative termination when power dissipation is a concern. Unlike other
termination types, Schottky diodes do not attempt to match the line impedance. Instead, they simply
clamp the over- and undershoots caused by reflections.

This image is an example of unclamped overshoots ...
Ignore the yellow line.

[Andy Wood]

Ok - got it now. Just a different method of achieving a similar result.

One has to wonder then why the terminating resistor is by far more popular? The diode arrangement seems more versatile and consumes less current......

Andy