# Trace Dimensions limitation for JTAG signals

I have a JTAG bus that I need to go over PCB (or could make cable) for about 12 inches. I am trying to figure out the signal integrity specs for the JTAG bus ... what trace width vs. trace length I can use and how much dB loss per unit length?

Trying to understand how long traces can be.

• Programming Altera PLDs with JTAG didn't work further than about 250mm on a job that I did and getting Altera to cough up the facts was a nightmare. That was about 5 years ago btw. Basic signal reflections was the problem. I never investigated it further because it wasn't an important product feature but I suspect some form of termination could be made to work. – Andy aka Jun 5 '15 at 21:50

The trace impedance is also not critical: As long as you maintain the impedance consistently, you will be able to deal with reflections. If you only connect a single JTAG target, you can get away with series termination of your signals. For a typical 50Ω impedance trace, add a ~30Ω resistor in series with the driving pin (close to the JTAG adapter for TDI, TMS, and TCK; close to the JTAG TAP for TDO). Of course the exact numbers depend on the ICs you are using and their driving abilities.
• Could you please elaborate on why in this specific case longer traces will not limit the maximum speed? For a simple synchronous JTAG implementation, I see a limit because of finite roundtrip time between TCK going out, and TDO coming back. – corecode Jun 6 '15 at 9:53