I'm trying to understand how a JTAG connector looks. To successfully interface with a board, are there any alternatives to JTAG?

Will all the pins of the JTAG be clumped together, or can they be spread apart, like in different corners of the board?

I have a complex board I'm looking at, and it has a lot of TP (test points) on the board. Can any of them, together, form a JTAG?

Will there be any marking on the PCB to help identify JTAG?

  • \$\begingroup\$ A collection of test points migth form JTAG, but if they're not labelled the only way is to trace them back to the parts. What processor are you trying to access? Can you post a picture of the board? \$\endgroup\$
    – pjc50
    Jan 11, 2013 at 15:38
  • \$\begingroup\$ The front of the board has a huge metal plate covering the whole board. Here's a blurred pic of the back of the board: dl.dropbox.com/u/19240828/Boardie.png \$\endgroup\$
    – Vegetto
    Jan 11, 2013 at 16:06
  • \$\begingroup\$ That's .. quite blurred. I can't even see a part number on the large IC. The cluster of 10 pads on the bottom left looks like it might be the signal pins of ARM JTAG: jtagtest.com/pinouts/arm20 , although there's also a cluster of 9 pins on the bottom right which might be the back side of some sort of connector? Of course, it could be under the shield on the other side. Is that a SATA connector on the top? \$\endgroup\$
    – pjc50
    Jan 11, 2013 at 16:21
  • \$\begingroup\$ Yes, the bottom 9 pins are from a connector, and the SATA connector is on top. How do I check if the 10 pins on the bottom left are sending signals? I only have a multimeter. \$\endgroup\$
    – Vegetto
    Jan 11, 2013 at 16:56
  • \$\begingroup\$ JTAG is an input, so even if it is JTAG you won't see anything. It might be outputting a constant voltage on Vref. \$\endgroup\$
    – pjc50
    Jan 11, 2013 at 17:45

1 Answer 1


JTAG is a port like any other (you also wouldn't want to have to connect five different cables to attach a LCD). The layout itself doesn't matter although there are various standards (probably every vendor has its own standard, or two) but since it is possible to also use a high clock frequency (especially on TCK - 25MHz) you might want to route the lines together and not spread them apart with different lengths. If you want to access them via testpoints, why not place these together. Also note that it's usually possible to make a chain of multiple jtag devices. You will need to connect TDO to TDI of the next device and so on. For the JTAG port you will usually have the following pins: TCK, TMS, TDI and TDO. Sometimes you might also want to have a Reset Pin where you can reset your peripherals but this is usually no JTAG requirement. You will have to check your devices if you need a reset. Check the usual suspects for their typical connectors (ARM, Xilinx, Altera). These are pretty well specified and you can buy tools with the appropriate connectors (instead of having to make your own adapters).

  • \$\begingroup\$ Are there any alternatives for the JTAG? \$\endgroup\$
    – Vegetto
    Jan 11, 2013 at 16:07
  • \$\begingroup\$ Depends on what you want to do :) \$\endgroup\$
    – Tom L.
    Jan 11, 2013 at 16:23
  • \$\begingroup\$ I'm just educating myself on how to interface with a PCB ;) \$\endgroup\$
    – Vegetto
    Jan 11, 2013 at 16:57
  • \$\begingroup\$ I see :) In that case you should find out which IC is on the board and which interfaces it offers. But if you only have a multimeter you won't get far. Also, the device itself usually doesn't transmit anything by itself. It will be you who has to transmit to the device and then hope it responds :) \$\endgroup\$
    – Tom L.
    Jan 11, 2013 at 17:06
  • \$\begingroup\$ Aren't there debug messages coming out? Can it not be tracked with a multimeter? Like short jumps and falls in voltages? I'll accept your answer for now, 'cause I think my question has been answered sufficiently well by you and pjc50 to give me a good picture of what to do. Thanks! \$\endgroup\$
    – Vegetto
    Jan 11, 2013 at 17:25

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