Stefan,
The inter-module communication bus is a custom bus that was designed and patented by National Instruments. Information on the bus and the bus components is not available.
From your description, it sounds like the FP-1000 is alternately recognizing and not recognizing a FP-DO-4xx module. There may be a number of different reasons that this can occur. Without more infomation on the specifics of your set-up and wiring, I can only provide some general ideas on causes.
First of all, where is the FP-DO-4xx on the bank? Is it the last module? If so, I would suggest that you make sure that vibration and/or a bent pin in the terminal bases is not causing intermittent loss of contact with the terminal base/module. One way to test this is to send the !B command (described in the FP-1000/1001 Programmers Reference Manual, http://www.ni.com/pdf/manuals/321632b.pdf ) through HyperTerminal (or a similar utility). If the FP-1000/1001 can detect the terminal base but not the I/O module, you will get a response indicating an empty terminal base. This will help to identify where the failure is occuring. Likewise, I have to ask, how is it mounted? If you are on a DIN rail, and have not used the rail locks, the bases may not be fit together properly and we may have intermittent signals.
Another thing to look at is the wiring to the FP-DO-4xx. What do you have the FP-DO-4xx outputting to? If the loads are highly inductive, turning off an output causes back-EMF that can sometimes knock a module offline (the module does have flyback protection in it, but sometimes additional external flyback circuitry is needed). For a diagram on using external flyback protection please see: http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/862567530005f09e862566a900660bda?OpenDocument
Finally, how is power wired to the network module and the I/O modules. If you cascade the wiring (by jumpering the V&C terminals from one base to the next, down to the network module), you power both the FieldPoint Bank, and all of the output signals from a single power supply. The power supply must be sufficiently rated for all the FieldPoint I/O modules plus their outputs. It may be with certain outputs turned on, you overload the power supply and some/all of the modules may stop responding.
Note: This method of wiring defeats all module to module isolation.
Regards,
Aaron
LabVIEW Champion, CLA, CPI