Re: [CAD_CAM_EDM_DRO] Re: Major encoder trouble, need advice.

Henrik Olsson wrote:

> Hello again,
>
> I've now tried powering the encoder with a separate 5VDC power supply, a
> 1000uF capacitor across the powersupply terminals and a 1uF MKT cap right on
> the encoder. That didn't help....
>
> I checked the encoder and there is NO termination across the signal-pairs on
> the encoder, perhaps that's why it seems to work "better" (but still not
> working) when I add "my" termination in // with the SN75115's internal.
>
>

Ok, there are a couple of things left to try. One is to put pull-up
resistors between each differential line and +5V. What voltage does the
US Digital encoder pull up to when set up as you have done it?
You mention you change the CABLE with the US Digital encoder. Is it
truly twisted pair? What is the rise/fall time of the encoder signals?
Is there any possibility the cable is cross-coupling A to B and
vice-versa? A short blip on A whenever B changes state is not unheard
of, and of course, that is an illegal transition by definition. If the
AVR micro happens to sample right then, bingo!

Parallel termination at the source is an odd thing, only used when you
have a bi-directional bus or expect very bad termination at the
receiving end. Series termination at the source end is a lot more
common. In this case, you'd put an approximately 60 Ohm resistor in
series with both differential outputs. When you do this, you stretch
the rise- and fall-times such that even with no receive-end termination,
you should not get much reflection. If your cable is highly capacitive,
it may be causing the differential driver in the encoder to "bobble"
sourcing the current to the cable capacitance. This could bobble the
other channel in the same chip. If the AVR is using an edge-detect
logic to sample on logic transitions, well, all I can say is I'm not a
great fan of the UHU designs.
I will also say that I have not been impressed with the low-end of the
US Digital encoders, and would
recommend Renco's new line of lower-priced units, which are just about
the same cost.

Jon

Jon