More on Re: Insulating D-shell from chassis? Xylotex board

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Moore" <pminmo@c...> wrote:

> Reading the Xylotex data and looking at pictures of the board, I

feel

> pretty confident of two items. One the board is based on the

Allegro

> A3977 chip, I also don't see what looks to be any other logic on

the

> board. Straight from the 3977 Data sheet - "to avoid problems due

to

> capacitive coupling" snip "always drive the logic inputs with a low
> source impedence to increase noise immunity". I'd like to buy a

3977

> controller board but have passed on both the Xylotex and THS board
> because they didn't have any logic on board driving at minimum the
> step, direction and enable lines. Resolved to designing my own.

Ah, the noise issue appears once again. The only way to prevent
conducted noise is to isolate all of the signals. If you connect the
grounds of two devices you have a possible noise conduit. Running
everything to a common grounding point is not always the answer
either. Most of the higher end driver units have opto isolated step
and direction inputs. Their internal circuit ground is not tied to
the PC ground and this eliminates one of the areas of possible
conducted noise and EMI. The more power you run and the longer the
wires the worse radiated noise problems you will have. Any wire of
any length is an antenna at some frequency or its harmonic. Try never
to give a circuit more than one ground path.

In running a larger plasma table noise is a challenge to overcome.
Not only do you have the motor noise but throw in a 5KW spark
generator as well. I had to make sure that none of the connections
from the table shared the same ground as the PC. The plasma unit and
table mechanicals share a low impedence (heavy wire) earth ground 3
Ft away. All of the limit and home switches run through Opto inputs.
The Unregulated DC does not share a ground with anything but the
Gecko drivers. The PC is grounded only through its normal internal
supplies. The Sound Logic board provides the buffering and isolation
to drive the GECKO's. The Torch Height Control has its own isolated
power supply and Opto's or relay contacts for its feedback.

By driving the motor controller with low level logic signals directly
from the parallel port and having a common ground (which you have to
in this configuration) you have a low level of noise immunity. If you
take a battery operated AM radio in the shop does the motor control
blank out a strong local station? If it does it would indicate
radiated noise (stuff going through the air). Bad brushes on the
motor can send out some pretty strong EMI.

Diagnosis and cure of noise problems can be frustrating without test
equipment. Even scopes can be useless unless you disconnect their
chassis from safety ground and use the shortest probe ground. I can
hear the grinding of teeth in the audience but we had to do this when
designing switching power supplies to get any meaningful readings.