I recently acquired a parts donor for my working 468. The donor unit had a GPIB card which I tried to transplant into the working one. Having checked the documentation, I found that a DIP switch enabled the GPIB option. This I located and turned on. In addition, the ?transmit¡¯ switch and indicator LEDs were missing. Fortunately I had a similar switch in my box of bits which I was able to install and I used two LEDs instead of a single red/green one. The first problem is that both LEDs are turned on at power up which I am not sure is correct? The second is that when I connectthe GPIB board, this prevents the computer from starting up. There are no flashing zeroes on the 7-segment display. Disconnecting the board restores normailty. I checked to make sure that the short connector ribbon cables are oriented correctly. Away from the scope I checked the board and there is no obvious power rail short. On an external 5v PSU the board draws about 300ma and none of the ICs appear to be getting hot, so I am unsure what steps to take next.
Can anyone help with what further diagnostic steps I might take please?
