It's not important that I understand all the details, but I don't see how it would be any worse to have a third or fourth independent signal to/from each board. You say they would be "interleaved" with the others, but you don't explain why that would be a problem.
You actually have 3 independent, non-bused signals to each board now.
Ok, that's what I would have expected. Are these modules pin out limited? How many of the general modules will the backplane have?
This problem is every time something is suggested, another requirement shows up, too "complex", no active circuitry on the back plane, too many pins on the CPU modules...
If I knew more about what you were already doing, I could know more about what you would find acceptable.
You are not happy with the ADC approach because the part is a couple of dollars. You might be able to use a similar, but less costly approach. Maxim makes a 1 wire device with two PIOs (programmable input/output) for slightly over a dollar. One bus line would be used to communicate with these devices. A second bus line would be used to sense the slot and board type.
Each sense line would have a resistor in series with the output on the backplane. The value of this resistor would correspond to the slot. This resistor value would be sensed by the CPU board by measuring the current when the plugin board pulls the card end of the sense resistor to ground with one of the PIOs. The resistor values would need to be small allowing a relatively large current to flow.
To determine plugin card type, the other PIO would be activated to pull the sense pin low through a larger value resistor. The CPU board would need to measure a much smaller current to distinguish the plugin card type. If the steps in resistance values of the two functions are distinct enough, nothing special would need to be done to make all measurements. But this would require very high precision devices. It would be better to allow the two ranges to overlap and do the math to isolate the slot from the plugin card type.
Or... the slot can be detected by measuring the current when the sense pin is grounded by the plugin card. The plugin card type can be detected by measuring the voltage on the sense bus when a voltage divider is enabled and the voltage source on the bus is disabled.
This may take a little circuitry on the CPU card, but it should be pretty minimal.
R1 Sense Bus Vcc -->>----/\/\/----+ ^ | | +------+ | Sense Pin | | SWa |----+------>>----/\/\/--+-----> to CPU
1 Wire | | | RslotN | sense circuit Comms ->>-| | < | | | > R2 | | | < | | | > | | | | V | SWb |----+ Sense Bus +------+ | VI can think of another approach that would dispense with the 1 wire device altogether, but it would be a bit more complex on the plugin cards.