JTAG Cables

Introduction

TAP adapter cables are often necessary to convert from the standard
pinout to the TAP connector pinout of a particular target. The pinout may be
Altera or Xilinx programming headers, CPU emulation headers, or other
proprietary pinout. In this discussion, we’ll cover design considerations for
creation of custom TAP adapter cables.

Creating TAP Adapter Cables

Most TAP connectors are two rows of 0.025 inch square posts on 0.1 by 0.1 inch
centers, making them suitable for mass terminated ribbon cable. In some cases,
the TAP connector may be single row, or part of a much larger connector, such as
a DIN connector.

When designing and constructing an adapter cable, there are a few design factors
to consider.

• Which Boundary-scan controller is being used? If only
  controllers with 20-pin TAPs will be used, a 20-pin ribbon cable connector
  such as a 3M 3421-6620 will plug directly into the controller. If an older
  controller will be used or a variety of controllers will be used, we
  recommend using a 10-pin cable connector such as a 3M 4610-6351 for maximum
  compatibility. This will accept the 10-pin cable connector from all
  controllers.
  
  
• Ensure that the mating connectors are obtained first. The acquisition
  process may take days, so get it started as soon as possible.
  
  
• Maintain good signal integrity by using as short a cable as
  practical. This will help EMI, crosstalk, cable capacitance, etc.
  


• Maintain good signal integrity with good signal return paths. The ground
  wires affect signal integrity because they are the return path for the
  signals. To enable high TCK rates, our boundary-scan controllers have signal
  slew rates in the 2-5 ns range. This requires a good signal return path,
  commonly called ground, to insure signal quality. On the standard
  pinout, there is a signal return path for every signal. Many TAP connectors
  on boards to not have a ground for every signal. We recommend connecting all
  the grounds of the boundary-scan controller cable to the target ground pin
  or pins. If there is one ground pin, it should be fanned out to all the
  cable grounds. If there are two ground pins, we recommend connecting the
  board ground pin closest to the board TCK pin to the cable ground wire
  closest to the cable TCK signal. All other ground wires in the cable should
  be connected to the second ground. For example, for the Altera programming
  header, the wirelist should be as follows:
  
  
  
  Table 1: Example Pinout for Altera Programming Header
  
  
• Maintain good signal integrity with signal termination. Serial
  and pullup/pulldown termination is best done on the board. However, if the
  board lacks the appropriate termination, it can occasionally be solved by
  placing the termination on the cable.
  
  
• Verify the pinout. It is very easy to swap the TDO/TDI pin
  assignments of the target versus the boundary-scan controller cable. Do not
  rely on the signal names. Check that the direction of the data flow matches.
  


• Test the cable. Once the infrastructure test is working,
  determine the maximum TCK rate. We recommend then looping infrastructure so
  it runs at least two minutes. This will test the signal integrity of the
  scan chain, including the adapter cable.
  
  
• If an adapter PCB is used instead of an adapter cable, the same
  concepts apply. Verify the pinouts. Use a ground plane to insure good
  signal return paths. Connect as many ground pins as possible to the ground
  plane.
  


• If the UUT does not have the recommended termination, it may be
  helpful to implement the termination in the cable or adapter PCB.
  
  


• A cable with a ground plane is usually not needed. If the signal
  return paths are limited, it may help, replacing the signal return paths. If
  testing in a high EMI environment, it may help provide some shielding when
  oriented so that the ground plane is between the EMI source and the signal
  wires.
  


• If in a high EMI environment, use twisted pair wires, preferably
  twisted, shielded pairs. This can be awkward to implement, so this
  is recommend as a last resort when EMI is a strong suspect as a problem
  source.
  


• When using wire wrap wires to make connections, the same concepts
  apply. At least twist the TCK with a ground wire. Preferably, twist all
  signal wires with a signal return wire. Connect the signal return wire
  at both ends. On the controller end, connect to the “paired” return wire
  (1&2, 3&4 etc). At the target end, connect to grounds as close as possible
  to the signal connection.

Source: JTAG TAP Adapter Cables