Unable to scan device chain

[Altera_Forum]

Hi all, 

I´m relativ new with the CPLD´s, and now I have the first problem with programming the device. It´s a EPM570T100. I ´m using a Elcamino PICoMax to programm the device. This works with one of my boards, so the programmer seems to be o.k. But I have another board which has always this error message when trying to Auto-detect:   Unable to scan devive chain. Can´t scan JTAG chain......   I checked twice every pin, VCC, GND and the 4 JTAG pins... no error found. I also measured with the Scope at the 4 pins. I see a signal on all 4 pins, but I´m not sure if they are right...  My three questions:    Any ideas what my problem is? What do I have to see at this pins? How do I use the debugger to find the error?   Richi

[Altera_Forum]

What devices are in the chain on the board that doesn't work ? Do any devices have a TRSTn pin ? If so, this pin normally requires to be held low for normal operation and raised high for JTAG chain. Holding it low will break the chain.

[Altera_Forum]

Hi, No other devices in the chain. 

But I fixed it:  1. My Laptop seems to have problems with the parallel port. The other PC works now 2. I had to put more caps to the supply. 4x 100nF are not enough. Now I have 6x 1µF and it works!

[Altera_Forum]

Good diagnosis 

 I've also had problems with ground loops and laptops. Some (perhaps older) laptops have 2-pin mains adapter connections, which means there is no earth. So a substantial AC leakage current can run round the JTAG circuit ground if the DUT is properly grounded. There are at least two ways to fix this: 1 get an AC adapter with a properly connected ground; run a substantial ground cable from the parallel port to the DUT's ground.

[Altera_Forum]

Just solder 4 x 10nF Capacitors from all 4 Data/Clock wires to ground and everything is fine!