Re: [CAD_CAM_EDM_DRO] EMC, ISA and other Questions...

alex wrote:

>OK, thank you very much for the explanation.
>I was looking at the C code of EMC but from the controller end of it.
>the reason I'm interested is because I want to mate DOS powered nodes with
>the central node, running EMC interpreter.
>Why I like DOS - not because I'm fond of typing away, but because it gives
>an ability of easy control of the hardware. In fact with Debug you can pry
>it
>up to the limit. I still cannot quite figure out , Jon, how you were able
>to control so many devices from the parallel port, given limited
>availability
>of the address lines.
>

There are no address lines. The RS-1284 spec renames the control lines
to be Write/, AddrStrobe/
and DataStrobe/. So, the 8 bits on the data lines when AddrStrobe/ goes
low becomes the address
to read/write data from. In this particular setup, it was most
advantageous to read or write
consecutive bytes, so I made the address counter in the devices count up
after each data transfer.
So, I have 256 possible device registers to talk to.

> On the other hand, little DOS modules will give an
>access
>to ISA bus with huge number of addresses.
>
>

Trying to keep a bunch of processors synchronized to the millisecond
level is not an easy
thing to do. I think keeping it down to ONE CPU is a great simplification.

Jon