wild I/O interface idea

[Doug Fortune <pentam-at-home.com>]

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.

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:

#1 - provides a few 32 and 64 bit timers/counters.  These are
          set up and read just by memory reads and writes to specific
          locations.
#2 - provides latched outputs to the real world (ie computer
         writes 32 bits to a memory address, and those are latched)
#3 - provides latched inputs to the computer (ie something
        external writes and strobes bits into a latch, and the CPU
        can read those bits just by reading a memory address).


#1 would assist RT-Linux in quickly setting up timers (I haven't
quite figured out how to do an interrupt, but perhaps cause a
false memory parity error, which could be trapped and intercepted).

#2 and #3 would give us all the I/O bits we needed.

Most modern (Pentium) computers have 3 or 4 SDRAM memory
sockets.  It would be easy to dedicate one socket for this function.

It seems much simpler than designing dedicated PCI cards, writing
drivers etc.  Also it would have a very wide variety of applications
for fast data I/O.

Sleeper, or just too crazy?

Doug Fortune