How does it work: Fluke 9010 Protection Circuit

[NivagSwerdna]

I'm toying with the idea of making an in circuit tester like the Fluke 9010 Troubleshooter Pods and looking at the original design for inspiration/ideas/plagiarism 



The Fluke Pod has the ability to sample the individual lines into a latch whilst driving/listening to them to diagnose failures such as stuck hi, stuck lo, shorted to another line on the same bus etc and additionally it has protection diodes , presumably to protect the pod.

The protection diodes are connected to 4.3V hi and 0.7V lo supplies; I presume that is to ensure the protected signal can go between 0 and 5V when the diode drops are taken into account. 

The 4.3V supply (pulled up by a 200 ohm resistor to 5V) comes from a now long obsolete proprietary IC; likewise the +0.7V (pulled down by 200R to ground).

Considering the +ve side... I presume the U10 device is meant to sink to keep the 4.3V at 4.3V even in the presence of a spurious voltage on the input. 

Now the POD specification suggests the protection is intended to protect at over/under voltages in the range +12V to -7V.

So... if +12V were applied to the input then the 'Sink' needs to dispose of sufficient current (limited by the 100R) to keep the uP input at 5V?

This is completely outside my knowledge...

How would I implement the Sink behaviour?  Sounds like the job for an Op Amp and a voltage reference?

And the source?  The same?

Thanks in advance

[NivagSwerdna]

So... choosing an arbitary diode it looks like I need to sink the best part of 100mA to keep everything in line; the original had +/-5V and 12V supplies; I will only have 5V.