Ellusive solution
Hippy, your reply is appreciated. Here is a little reading.
Initially doing pulse width frequency conversion with pulsin and pulsout commands looked like it would be a snap with just the commands I needed. Unfortunately those commands are exclusive of each other and with other commands. After considering a handful of approaches (see previous posts), it looks like the suggestion to use a second order passive filter pulse width to voltage conversion is a simple viable option. There is a minimum of external components, but some concern about the lag - ripple trade off. For faster response I worked up a pulse width to voltage sample and hold circuit, clocked by the input pulses. This circuit required a fair number of components, and has unresolved reset issues if no input pulses are present.
Also considered using ADC to monitor an external 20Hz. saw tooth ramp - time base for the input and output pulses. It looked like it required some convoluted programming, had poor resolution for short pulses, and could have missed pulses due to processing time. It galls me to think digital I/O's require an analog interface.
It appears multiple 08M's could have communication issues, especially in conjunction with pulse commands. Other approaches have been discussed here, and generally have had one issue or another.
With most any input scheme, the ~ 20Hz. output pulse looks like it should be made from 2 output pulse commands. One for mark and the other for space, with the sum of their time = ~1/20 sec. This doesn't leave much time to do anything else, like A/D and maths, without being part of a mark or space time.
An accessible counter(s) sure would be nice, but then again that's kind-of what pulse commands are except one command hogs the timer. If there were a way to run 2 pulse commands concurrently, this project would be done, even if time resolution was reduced. How about a small PicAxe with more counters and/or pulsin and pulsout sharing a counter. This might help others with pulse - pwm command issues.
Again, it is perturbing to think that a project with 2 pwm inputs, some maths, and one slow pwm output would require analog circuitry or more than a 8 pin micro.
At some point, all this casting about leads to looking for something more suitable. Avoiding the resulting undesirable learning curve is an issue.
I was delighted to find and work with PicAxe, continue to be a fan of PicAxe, and I have recommended it to a couple friends. It can readily do other projects I have in mind, so it will be the first choice. If it would just do this darn pulse width conversion thing.