I'm building a JTAG programmer for the old Motorola 56k devices and I want to add some circuit protection before having some boards built. I'm more of a firmware/digital guy, but here's what I came up with based on this TI whitepaper. However, I was told it would not work because (I believe, this was a few weeks ago so my memory is foggy) because I wouldn't get current to turn on/off the transistors.
The idea is not necessarily to protect against the technician from reversing the programmer orientation, but rather to provide some protection in the case that the device being programmed is powered backwards. So, in other words, in the case that the 56K is powered backwards (something I'm guilty of doing because the wire colors were reversed on the unit I was given), it would more or less damage the programmer too.
So the idea is that
pins 4,5, and 6 should normally be connected to ground and
pin 11 should normally be connected to 3.3V. But if the polarity is reversed on the candidate device, the opposite would have happened. In this case, my hope is that both the PMOS and NMOS transistors would be turned off, thus preventing current flow. Based on the above diagram, is this true? Is there something else I need to do to make it work? Perhaps switching out the transistors for relays to avoid the aforementioned current flow issue?
Here's a higher-level diagram of what I'm trying to do.
Of course, assume various DC-DC converters are present in the DUT to step down from 24V to 3.3V