Why is 6533 burst mode erratic when _5V not terminated?

[ajd]

I am using 6533 10Mhz burst with SH68D1 cable 5 meters long. I do not use the +5 from the 6533 in my unit. When I connect the cable shield to my case ground, the 6533 becomes erratic and it reads randomly, usually it reads the data from my FIFO 32 - 32bit words at 1khz rep rate. When I touch the +5V pin1 with anything (finger, scope probe) it works OK. +5V comes down cable to my board and it is not connectedto anything.

[rgharrison]

Hi ajd,

It sounds like high-speed termination issues. Typically, when high-speed signals > 1MHz travel down a long wire such as the 5M cable, reflections could occur if the signal is not terminated correctly. The situation that you are seeing is that you are terminating the 5V line with a high impedance such as your finger or the probes and this stabilizes the 5V line which in turn can be stabilizing your other digital lines. Your 5V line should be stable but it is probably picking up signals along the cable and thus is becoming a high-speed signal itself.

The best easy solution is to terminate your signals with a high-impedance such as a large resistor. However, what you are seeing is not uncommon when dealing with high-frequency signals in a l
ong cable. I am attaching an excerpt from one of NI's documents regarding high-speed effects. Hope that helps. Have a good day.

Ron