For JTAG / Boundary Scan applications driven externally by an automated test system (a desktop PC, or a laptop for field service, for example) - as opposed to embedded test controllers - the Unit Under Test needs to be connected to that test system either through a test bus cable or through some sort of connector or probe based test fixture.A test bus cable should not be limiting the test systems performance.
Sometimes, test bus cable design introduces noise or otherwise impacts signal transmission quality negatively. This could result in a lower maximum TCK frequency than what the scan chain(s) on the UUT support, or even - in the worst case - render a Boundary Scan test unreliable or impossible. Several measures can be taken to avoid problems associated with noise and reflections on test bus signals. One very simple remidy is to create a twisted pair design for the test bus cable, in which the JTAG signals are paired with ground. This approach, when combined with good signal termination on the UUT in most cases asures that test and in-system programming applications can run at the maximum TCK frequency supported by the IEEE 1149.1 compliant devices on the UUT.
And example test bus cable design is shown below:
In this layout, each test bus signal is routed next to a ground line in a twisted-pair design (e.g. using twisted pair flat-band cable, or perhaps even CAT-5 cable).
