Terminating Resistors

[TimeFluxCap]

What purpose does the 150 ohm resistors at each end of a DH+ network perform?

 

[Gerry]

What I can remember about transmission line theory

It's been a very long time...

The blue hose (transmission line) has a characteristic impedance determined by its distributed inductance and capacitance. Presumably, the characteristic impedance for blue hose is 150 ohm. If you inject a signal into a line that is terminated with anything other than the characteristic impedance, you will get reflections. Low values give negative reflections and high values give positive reflections. The reflections interfere with the injected signal causing attenuation or complete loss at different points along the line determined by the wave-length (frequency) of the signal. You should note that if the line is pinched, kinked, or otherwise damaged anywhere along its length, the point of damage will also cause reflections.

Now, if somebody could explain why the termination should change to 82 ohm when operating at 230K baud....

 

[okiebob]

The hose

Also remember to NOT pull the wire ties to tight. The crimping of the ties, the lay of the wire (coiled vs. looped) affect the H and the Z of the run. Short runs (under 100m) not a big deal, BUT long runs and you could have probs. See Pete Stein and Pat's page for measurement Systems Engineering for more info.

 

[seppoalanen]

Originally DH was don for 75 ohms coax-cabling. Best result is when impedance is 75 ohms so. 2 pcs 150 ohm resistor. Short Trop-lines are possible but not terminate them.

Now, if somebody could explain why the termination should change to 82 ohm when operating at 230K baud....

If frequency is higher, capacitive reactance between pair-wires is lower so it naturally lowering impedance if cabling is wide. Then terminating resistors (2 pcs) must be bigger than 150 ohms.
That is just theoretical el-technics.