I've traditionally been using an I2C master hub for development, now I want to build the simplest, cheapest possible I2C connection imaginable for PowerBasic DOS output. I think the best design I've been able to come up with is two resistors, two zener diodes drawing +5V off the serial port (.50 cents worth of parts). This will work on machines with standard RS-232 serial, but might not function correctly on some laptops or where the device cuts corners on the DB9 port. In the post-apocalyptic environment, I don't think the design of this circuit would be difficult to replace for anybody anywhere. A dollar buys enough replacement parts for this circuit to build fifty of them. It's not even necessary to use a soldering iron, this basic circuit could be wire-wrapped.
The great thing about the serial port is all the cheap optical isolators that are available for it. I intend to provide drivers for a parallel port I2C interface but I will make the DB9 the "de facto" standard for Vault-OS so anybody can build it in ten minutes if they need to using a serial port cable.
Anybody who can point me to the best design they have seen for I2C through DB9 RS-232 is invited to drop me a link. What I have is working but is not safe until a commercial optical isolator ($20-$30) is connected between the hardware and the I2C output. It has to be EMP proof.
There are parallel port designs with optical isolation built into the circuit but trying to make LPT the Vault-OS "standard" for I2C would be a bad idea since they are getting to be pretty rare legacy ports.
My basic stance in hardware design has been to use DB9 RS-232 as the ultimate failsafe for peripheral control and to use the parallel port (when available) only for crude relay switching circuits which could be constructed from automobile relays ripped out of abandoned cars. If Vault dwellers can't scavenge up a few car relays or a couple of diodes/resistors from the wastelands I can't help them.
This is an expression of the VOS philosophy to keep the entire core system so simple it is foolproof, decentralized, independent, asynchronous and completely maintainable indefinitely.