Legal Removal Request

Post: Sorry for not posting more frequently. I'll try to do better, but I don't want to burden the AB Electronics board folks with a bunch of drivel about code when, after all, they're in the electronics business. I'll try to off-load that sort of overly-detailed info (a character defect on my part?) to a website that I'm developing, [url=https://www.erlambert-phd.com/]ERLambert PhD[/url]. It's a work in progress, so if you visit, please bear with me. As I left off, I was developing a graphical interface for RasPi and their AB-E expansion boards. It has gone great, and I am able to put up a track diagram, control the power distribution via the AB-E boards and added relay boards by clicking on the diagram, and show status on the graphics, all via C using GTK GLADE code. Outputs: Cool! I'm now in the portion where I need inputs. I have a pair of very reasonably priced 8-signal PC boards which I bought via Rob Paisley's excellent Canadian website, Circuitous Circuit Index. They're well-designed, and they work! Optical transistors mounted just below the rail ties (that's "sleepers" to you UK guys) turn signals off and on. They output 12-volt signals, in keeping with model railroad tradition, so I've added two opto-isolator boards which convert the output to 5 volt signals for all 16 detection positions. Here's the problem: the isolator boards' signals drive on-board LEDs, and all the track position detectors work great, but if I plug the 5-volt signals to the AB-E board and set (or not!) the board for input, all but one of the LEDs remain dark, no matter what the signal state. Unplug the input signal header, however, and the LEDs all function as desired. So, my question: should I insert resistors between voltage converter outputs and the AB-E board inputs? I suspect I'm either over-driving the inputs or there's enough current demand that it pulls more than the 5-volt power supply can produce. Can anyone recommend resistor values? ...or is this the wrong approach?