Many here will be familiar with the great little portable TIU port tester that Adrian did the basic design for, many contributed good ideas, and finally grj did the board design for. It was developed in a topic called $10-20 DCS-TIU Port Tester Tool; and it is being distributed by rtr12, and it works quite well. Thanks to Adrian, Stan, grj, and others, and rtr12 for doing all the heavy lifting to make this kit available!
When I reread the thread recently I was reminded that someone had suggested early on adding an LED to the output of each TIU channel to give a visual indication of DCS output. This would function like the red LED on Legacy and TMCC bases, which is a very handy tool for troubleshooting.
I figured what if a simple 2 port led board could be easily morphed from the basic kit design, and could be designed to mount neatly inside the TIU case, with just the indicator LEDs sticking out through the cover, one near each TIU channel output? A 2 port device would be useful if you are only using the two fixed DCS outputs say. If instead you are using all 4 TIU outputs, you could install 2 modules and cover them all.
There would only be two internal connections needed per channel, connected to the respective port terminal posts inside the TIU. Output channel power would be adequate to power the board, just like with the tester kit.
I think many DCS users out there would be interested in such a device; it sure has appeal to me. It would be nice to make it through-hole design, like the portable tester, so anyone can build it. With any luck we can keep the costs down to about the same as the portable tester kit.
I am thinking only a single led would be needed per channel and the threshold signal level might want to be set at about 7-8 volts or so. That way any TIU kicking out the normal 10-12 volts or more will show an LED hit with every data packet. Ports with an output much less than 8 volts won’t activate the LED at all. If you are monitoring all 4 ports of your TIU, and suddenly one is not indicating when the rest are, you immediately know you have a problem. Sensitivity adjustment would be via a separate trim pot for each channel. So threshold voltage can be user set to whatever you like.
The portable tester uses 2 channels of a CD74HCT123E chip; one for the red led and the second for the green led. But we can use these same two channels for two separate TIU ports instead.
I have poked around inside a Rev I3 TIU and it looks like there is space for two 22 x 76 mm pcbs along the inside edge, end to end, near the output ports. If using only one, it would straddle the two center fixed outputs. If using two, each would straddle one variable and one fixed output, mounted end to end.
The pairs of TIU output posts are spaced about 56 mm C-C apart, so the two leds (one per channel), would be about 10 mm from the short ends of the pcb, and very near the edge closest to the outputs. If the leds are close mounted to the board, they can be glued into their holes in the TIU cover, to provide mounting for the board. A few details need to be worked out yet, but it looks doable.
The module includes a 3 pin Dupont type header to allow selecting either TIU channel for module power. This would allow versatility of hookup regardless of how many, and which, TIU channels are being used.
Here is the circuit I came up with:
To save some board space I deleted 2 stages of the original RC filter network, because I noted when testing mine with a scope there was no difference in output signal after 2, 3 or 4 stages. I have tested this with the portable tester and it behaves exactly the same as with all 4 stages. I kept the original blue power indicator led circuit just because it's handy to have.
I have landed on a preliminary board layout, which needs a few tweaks before I order some test boards.
So that's where I am with this project. I just wanted to test balloon this to forum members to see what level of interest there is, and collect any suggestions and comments you folks may have. 
It's probable that I will soon go ahead and order up a batch of prototype boards, just to try the whole thing out.
Cheers, Rod
