HI Charles,

On 3/11/2011 3:21 PM, Charles wrote:
Has anyone ever used this method for field troubleshooting?

I doubt you will find anyone using a *raw* JTAG i/f in
this way. You would need far too much information about
the devices involved, their interconnects, etc.

When *designing* a product, careful consideration to the
JTAG abilities of the device(s) you use can help bring *up*
a system (virgin hardware) by giving you control over
nodes that would otherwise be difficult to manipulate.
Many MCU's also use JTAG to program onboard memory and/or
a crude software debugging interface.

But, unless the manufacturer has specifically designed a tool
that hides all of this detail from you and reduces it to
a set of indicators (or, perhaps, an interface to software
running on a laptop), chances are, it won't be of much
practical use (though it is often used to "hack" or
"repurpose" existing devices despite the manufacturer's
original intent).

Some products will bring JTAG pins out to an (undocumented)
connector. Or, they'll be microprobed in production (to
save the cost of the connector : ). Similarly, many products
have "free" serial ports that are routed (and often have
interesting capabilities! : )

But, JTAG isn't as nice and semi-standardized as OBD, OBD-II,
etc. for automobiles... too many variables to account for.

Sorry.