You may experience intermittent JTAG communication failures if you are downloading, debugging, or communicating with Nios II systems over JTAG using the USB-Blaster™ Rev A, ByteBlaster™, ByteBlasteMV™, ByteBlaster™ II, or MasterBlaster™ cable. The modes of failure are:
Device Configuration Failure - Device configuration fails with an error message from the Quartus II Programmer
Debugging Failure with the Nios II IDE - Debug session terminates with JTAG I/O error
Nios II JTAG UART Failure - Invalid characters transmitted and received via Nios II Terminal
SignalTap Failure - SignalTap session terminates with JTAG communication error
The problem is noise related. Therefore the errors may occur frequently in a noisy environment and are more likely to occur the longer the connection has been established. The USB Blaster Rev B. (or later) has been designed to address this issue and will allow you to communicate successfully with Nios II systems. Revision B cables are clearly labeled as Rev B. (Revision A cables have no revision label.) Please refer to the file errata.txt included in the Nios II development kit (<Nios II installation>/documents directory) for details. Please note that you may still get benign warning messages if using a USB-Blaster Rev B cable shipped before January 30, 2005 due to a known issue with the identification firmware. These warnings can be ignored. USB-Blaster™ cables with the gold-colored flexible PCB cable are the solution to the JTAG communication failures.