We are talking about pain, not a pain signal. Decrementing register can be a pain signal, but it can't produce feeling of pain (qualia). If humans can be simulated on a Turing machine, we can't feel that definition of pain too.
If we make a robot sufficiently similar to us, it would indeed feel pain (qualia). The real question is: Where is the complexity/similarity treshold?
They obviously can't feel pain. Imagine you simulate that robot with a turing-complete cellular automaton on paper. At what moment in time will it feel pain as in qualia? When you scribe the next tick on a paper?