How to abuse JTAG specs

My roommate, surgeon
Russell, was having some difficulty programming a JTAG board. The board was designed some time ago when the JTAG specifications were gratuitous and claimed that an infinite number of devices could be chained together and programmed in serial. Over the years the JTAG spec has shrank from 20 to 10 to 5-6 devices on a single chain, so most of JTAG programmers probably only allow enough fan-out to support the spec, save a few pennies, right? As a consequence, these large boards with 15+ devices sometimes could not be programmed properly with modern programmers which sometimes resulted in setting the on-board chips in such a condition that cause permanent damage and required high pin-count device replacement. We discussed the problem at length over a few drinks and concluded that it was indeed a fan-out problem which could be solved by placing a pull-up resistor on the output of the JTAG programmer and then a voltage-follower that can supply enough output current to drive the chips. After building a prototype, it seems that the solution worked. Not a bad way to spend a few dollars to save a few hours of pin-lifting and chip replacement.

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