gunrunnerjohn posted:A second car would knock it down some, but remember it's basically a voltage divider, so when you start with a lower voltage and divide it, you don't have the same absolute change. OTOH, just seeing it go from 15V to 4V with one car suggests that the subway with multiple cars would be a major issue for DCS.
One question that should be asked here, is the voltage drop we're seeing with your measuring board the same as we'd see with the TIU? I suspect that the TIU may have a lower impedance driving the track than the board with 100 ohms in series with the output.
Adrian, how close does this track to the behavior of the TIU signal?
The TIU is 4 channels of 50 ohms (ACT244) ganged together in parallel so like 12 ohms, then a sqrt(2):1 transformer, so like somewhere around 24 ohms total. This one is set to 100 ohms so you don't blow the 7404 which is not a large quad driver like the ACT244 line driver. I thought about replicating the output stage of the TIU, but then you'd need to get the same transformer (which is a pain), and go surface mount for the ACT244 driver (which is a pain), and it didn't feel necessary just to figure out who's trains are adjusted and which ones still need to be done. I do have gain/offset trimpots there so you could turn the gain below 1 to try and match the load line of the TIU. That's referred through the diode detector though so you may have different shaped IVs and would only be able to match over a small range.
Despite the difference in gain, it is linear as a divider. A train that reads 5V does present have the impedance of a train that reads 10V. Again just trying to sort out "good" and "bad" quickly. If you want precision, definitely stick with the scope and look at the packets directly. Also note the DCS deocoders fail gradually. When the excursion is like 5V everything works fine, by like 3V you struggle with the long packets that add engines but still have short packets work like whistle and speed. At about 1.5V it's non-functional.
