I bought a used Easytouch PSL4 system online that came with a universal outdoor controller (uoc) and a universal indoor controller (uic)... and since I needed an Easytouch 8 uic for my test bench, I figured I'd try and hack the PSL4 unit into an ET8. I knew the outdoor boards use the same basic hardware across all versions but I'd never bothered to look at the indoor controllers. I took apart the PSL4 and laid its main board side-by-side with one from an ET8 indoor controller... predictably, they both had the exact same hardware. So how does it know it's a PSL4 or an ET8? Here's the Software Revision of the unmodified PSL4 uic Sometimes it's just a simple matter of swapping firmware, but hardware designers often employ a hardware-based means of identifying the version, which then dictates what features the software allows. In the case of the indoor controllers, the main boards are exactly the same - no different resistors or jumpers - but I did notice s