ISP, viewed from the production standpoint is a slow and therefore costly method of programming. While it's invaluable for field fixes, upgrades, etc, for manufacturing it's a millstone about one's neck.

Further, if a different tool is needed for ISP'ing each variant one uses, one quickly has a box full of programmers. That's why people like "universal" (they seldom are) programmers.

I've seen several of these "universal" programmers that actually use serial programming to handle newer and serial-compatible parts over the past 15 years or so.

If one were to think "parallel programmer" and "ISP" at the same time, one could implement a relatively flexible set of hardware, but making the software work with it might be a mite tricky. A "configuration plug" approach in which one applies a labelled plug to a universal socket of, say, up to 68 pins might be workable. One would insert the configuration plug between the parallel port or serial port and the on-board hardware such that the signals are appropriately routed. That way one would have a single device that handles multiple devices. That would allow for multiple programming circuits to be combined into a single device.

RE