Re: wild I/O interface idea

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Doug Fortune wrote:

> Regarding the previous ISA/PCI/USB/ethernet etc
> conversations on how to get signals out to/from the real world
> in a timely manner for steppers and servos.
>
> The problem with ISA is that it is constrained to
> operate on an 8MHz cycle.  I believe the accesses to the
> parallel port and system timers (for RT-Linux) also
> must pass over this bus.

No, the timer chip (assuming this is the system timer) is
on a motherboard chip set, and should be at near processor
speed.  If the parallel port is on the motherboard, the same is
true.  If the parallel port is a card plugged into the ISA, then it
will be restricted to ISA speed.  Since practically all Pentium
motherboards have at least one on-board parallel port, that
one should do fine for the real-time motion control.  A slow
parallel port could be used for the auxilliary controls.

Note that the STG card is an 8-bit ISA card, and performs
quite well, with servo update times of 1 mS or less.

>
> Once upon a time I built an analog/digital converter
> for an Atari/ST and it did its I/O through the cartridge
> game port - essentially memory mapped I/O at memory access speeds.
>
> I am thinking of a card, which to the computer looks like a
> SDRAM memory stick, but which actually is not memory,
> but otherwise provides three basic functions:

No, you can't do this on a Pentium, or other cached CPU, as
the memory location may not be updated for seconds after the
CPU executes a write to memory command.  Also, the Pentium
usually spreads memory access across several memory SIMMS,
although the latest 168-pin DIMMs get a doubleword access all
on one DIMM.  The other problem is designing a device that
can operate at 133 MHz bus speed, which is what the new memory
is clocking at.  I wouldn't want to do this!  Can you design an I/O
chip that can handle 2 I/O commands separated by only 7 nS?

Jon