Reinventing the TVM wheel

[Ángel Martin]

Being an old-school engineer has good and bad things, I 'sppose. Amongst the good ones is keeping a safety distance from the financial stage and associated frills; for instance the TVM implementations. But no more - finally my curiosity was piqued enough to take a peek at the insides of the TVM, with the goal being a full 41 MCODE version (what else...)

I should have known there was a reason not to touch that with a 12-foot pole - no doubt you all know about the intricacies involved in solving for the interest rate the equation, so it won't come as a surprise that after successfully solving the other 4 cases (N, PV, PMT and FV) I'm now stuck.

Not only the Newton convergence is extremely critical, but for the first time ever my trusty 13-digit OS routines are acting up:- either I've completely forgotten how to use it, or there are bugs in them (and either option is equally disturbing)...

There's very valuable information in this thread, but I can't find any follow-up in the archives anymore - even if I remember reading more in the forum. Would you point me in the right direction? I'm hoping somebody has solved the riddle (which is obviously possible, just refer to the HP-12C for the perfect example with a coconut CPU).

Thanks in advance,
'AM

PS. In case you're interested see the blueprint below - it has all five functions but the interest rate is still a FOCAL program... still a work in progress.