So I was sorting through my boxes of cables today, when I found the DE-9 to DE-9 monitor cable + DE-9 to DE-15 adapter that I had used many years ago to connect an old EIZO monitor to VGA cards.

That monitor only had a DE-9 input on the back but would accept both VGA and TTL signals over this connector (there was a mode switch on the front).

It is long gone now and I don't have any other TTL capable display either, but do have some TTL graphics cards, so, when I saw the cable and the chunky adapter,
I wondered if it would be possible to build a TTL to VGA converter into the shell of the adapter. Just something simple to get the voltages down to a level my OSSC can work with.

I opened up the adapter and found that there was already a tiny PCB in there with four three-legged blue "thingies" in the middle ... according to the markings on the PCB, the blue thingies are sitting between the R, G, B and H wires, the middle pins of all four thingies are connected to GND.

If I put a multimeter on the two outer legs of a thingie or the corresponding pins on the connectors, the meter shows low resistance.

Does anyone know what this PCB does?

Is this some sort of "voltage clamping" circuit to protect a VGA monitor in case someone connects a TTL card ... maybe some zener diodes coated over with blue epoxy ?
If it were to protect the VGA card in case someone connected a TTL monitor to it then there should be some measureable resistance between the two connectors - no ?