Re: digital servo controller to offload the Parallel Port?

[Matt Shaver <mshaver-at-erols.com>]

On Thursday 19 December 2002 12:24 pm, John wrote:

> IMHO, Jon's PPMC is the right direction as far as the interface to
> the PC is concerned.  The EPP port gives fairly fast and simple
> access to external hardware.

The only hitch (and Jon can confirm this) is that each "enhanced parallel 
port" chip has it's own peculiarities when it comes to shifting to EPP mode. 
I know Jon wrestled for a while with this and I don't know if he ever came up 
with a universal method that works for all ports. Laptops in the distant 
future may omit the parallel port, but they'll probably last longer than 
desktop PCs since travellers want to be able to print wherever they go. For 
desktops, the best bet is what JohnK suggested, use PCI parallel port boards 
(Siig is nice).

> The G2002 currently includes a
> microprocessor, which I don't like because it has it's own software
> to be written and maintained.  There is a Yahoo group called
> "geckodrive", and several recent posts there discuss how the G2002,
> with or without it's micro, might work with EMC.

It's also USB. It has plenty of bandwidth, but it's non-deterministic if more 
than one peripheral is on the bus. The other problem is that you only get one 
bus turnaround/millisecond/endpoint and I don't know if it's possible to use 
multiple endpoints with the USB hardware in the G2002. Since all 
communication cycles are initiated by the host controller in the PC, the PC 
will send out a velocity command first, then read the position data that 
resulted from the previous velocity command. I guess after the first 
command/position exchange, it will be OK (we'll see...).

Ethernet, direct connected to the PC through a crossover cable and using no 
communications protocols, just raw frames, maybe that's the way to go...

> (DDS instead of divide-by-n)

Also, if you're looking for a nice DDS chip, check out the Analog devices 
AD9833. It's serial loading, and you can bypass the sine rom and output a 
plain square wave clock which saves you from having to "square up" the DDS 
output. It's a preliminary datasheet now, but by the time any of us ever get 
around to actually using one of these, it'll probably be obsolete ;) .

Matt