microprocessor question

Hi Guys,

  I'm working out the specs for a microprocessor pair that will be part of
a PLC (Programmable Logic Controller) system. One of the processors will be
the main CPU for the system and the other will handle things like power
distribution, I/O control, A/D conversion, etc.

  I am also writing a control program, not the OS or the command
interpreter for the system, but a seperate program that will be downloaded
to the PLC and will execute from within the FLASH memory of the processor.

  Here's my question, I'm not sure how big my control program will be just
yet but I would like to determine the specific requirements for things like
processor clock speed, onboard memory, onboard FLASH etc... so what else am
I missing here?

  The PLC has to be capable of real-time processing, so the clock speed of
the main  processor should allow for that, any suggestions here? Minimum
value suggestion?

  Also, the PLC needs enough memory to do everything it needs to do. The OS
for the PLC will be 32-bit as will the control program, so anything more
than 4Gb is pointless. So what about the FLASH memory where the control
program will be stored?

  Other than the size of the program that will be stored there, which I'm
not sure of yet, what considerations should be made when determining how
much FLASH memory to build into the design? Thanks guys.


John