Byte Blaster MV cable Length

[Altera_Forum]

Hello, 

 I have ByteBlaster MV from Altera,Its not working.The error in quartus is " Unable to Scan the chain" I suspect the problem is in JTAG FRC Cable(10 pin). Can any one tell that what is the ideal length of this cable,Because i am trying to increase the length upto 26-28cm.   Please suggest me whether i am doing correct or not.  Regards Kiran

[Altera_Forum]

Is it working with a shorter cable ?

[Altera_Forum]

As far as not being able to scan the chain, if you are using the original cable, the issue may be the blaster itself or the board. As Woody said, try it with a short cable first. 

 About the ideal length, the short answer is: as short as possible. If you extend the length of the cable, you will run into reflection problems due to the lack of reference ground in the ribbon cable. As you make the cable longer and longer, you will start seeing bad overshoots and undershoots - it does not take a lot of cable to start hitting overshoots of more than 1V. I had that problem with a byteblaster and ended up opening it up and changing the series resistors on TDO, TCK, and TMS to help dampen the signal. After that change, I used a 20cm ribbon cable to achieve tolerable reflections.  You can experiment with different lengths if you have a scope. Measure the TDO, TCK, and TMS at the FPGA and see what kind of signal issues you get.  Finally, yes, JTAG will fail if you have serious signal issues.

[Altera_Forum]

Thanks woody and Ironmoose, 

 In Short cable it was working well.When i increase the flat cable length and kept for connected blaster for 25-30mins. From thereafter its not scanning the chain. In my board 100E Resistor have been used for TCK, TDO, TMS, TDI Whether i need to reduce this?  regards Kiran

[Altera_Forum]

Before running the JTAG with the long cable for a long time (30 minutes), I would check with a scope to see what your signal looks like. If it is overshooting into the 5V range, you will end up damaging the FPGA. 

 On the blaster side, you will need to have series resistors on TDO (blaster to board), TMS, and TCK. I would start with 33 ohm.  Of course all of this depends on the type of cable used and the length. I am not sure how you have your 100 ohms connected on the board side. Are they in series? How are they connected?  Having series resistors on TDO (Blaster to Board), TMS, and TCK at the board side, will not help your signal issues. Having a series resistor on TDO (board to blaster) at the board side will reduce over/undershoot seen at the blaster.