I've noted that, among the various applicable application notes, there are a couple of different serial programming circuits that have been published. While the differences are probably insignificant, in view of the problems amply described with ATMEL's ISP circuits, I think it's appropriate to call attention to this detail.

Some of the app-notes and datasheets have only to two of the MCU's signals, namely RESET and nPSEN driven, through 74HC/AC125's and enabled by DTR, while others have those and nEA driven through the '125, whereas the ones using only two of the tristate buffers drive nEA directly with the DTR signal from the DS232A.

I personally favor using all three tristate buffers, as the nEA may be driven with something else, perhaps.

I don't see the expenditure of the extra buffer as a major drawback, as there are four of them in the package. However, if you're not driving the nEA pin with anything else, and haven't got it tied to GND, then you can probably drive it with DTR in the way that's shown. However, if something else is used, you might want to consider other options.

RE