How have you connected to this thing? It has a serial interface, so there’s some box in between. Is that not giving you a gateway function?
Anyway, working wih bytes and hex in DSL rules is not at all easy. Your register 0E is in “two’s complement” signed form, where FFFA represents integer -6
This post looks helpful
var input = "FFFA"
var result = Integer.parseInt(input, 16).shortValue() as Number
logInfo("test", "parsed " + result)
logs -6, seems to work