It appears what you are asking could be accomplished by a
Microchip PIC device. Go to Microchip's web site and look up
the specs for the 16F628 chip. It has three internal timers
that can be slaved to an external oscillator. Two of the timers
TMR0 and TMR2 although they are eight bit timers can be quite
versatile because they have pre and post scalers. In effect
they can count to 16 bits. TMR2 is a 16 bit timer. They can
generate interrupts and TMR2 can interrupt at a loadable value,
PR2. And the 16F628 is dirt cheap. Costs less than a couple of
LEDs. There may be other PIC chips that could be better suited
for your purposes, I'm only familiar with the 628. Oh, the 628
has a built in USART capability which can communicate with a PC
via a serial port.
I'm working on a 16F628 electronic ignition project where I use
TMR0 to count the time between revolution pulses and TMR2 to
count down the necessary spark delay time. The beauty of using
these timers is that once loaded and started that keep on
counting regardless of what the rest of the software is doing.
They just keep on ticking.
Of course to utilize all these features you have to roll your
own code and then download it onto the PIC.
Yes, I believe hardware timers is the only way to provide steady
pulse trains regardless of all other code execution. I'm looking
for the minimum needed to provide three variable pulse trains
controlled by the PC serial port. What about using a Basic stamp
(easier for me) for the serial stuff and separate chip with 3
timers? What timer chips should I be considering? Someone had used
an 82C54 with a stamp. I've been fooling around with CMOS 4000, so
what do I know. They do function well for low freq stuff, but
changing divider outputs on a breadboard is getting tiring and I
can't keep track of what changes are made verses time. It's like
the stone age here.