Mesa Encoder input-index signal always high, even with grounded input

If pin 13 is damaged on the 5I25 it would prevent the last 7I74
channel from working (though it may still work as it's the TXEN
line and if its stuck low, the sserial channel will still work)

Typically if 5I25 inputs are damaged, it means they have been
exposed to negative currents >50 mA. This will usually short
out the clamp diode on the bus switch, leading to a stuck low pin.

This can happen if you plug in the DB25 cable and this connection
joins two systems with different ground domains (say a machine tool and a PC)
This also why Pin 13 and pin 1 are common victims (they are often the first to connect)

I power all my external cards 5V input through a power supply in the electrical cabinet (the W1 jumpers are set accordingly).
The 24V ground is connected to protective earth (original Hermle installation). I connected my 5V ground to the existing 24V ground.
Would it be better to use a separate 5V ground to protect pin 1 or any other pins in the future?
Or is it the other way around and the ground of the PC internal 5V needs to be connected to the common ground?

Its more that all power should be off on both the machine
side and PC side when connecting the DB25 because of
possible ground differences. The 5I25 is likely fixable by
replacing the bus switch (74CBT16211CDGGR)
 

That explains it, I assembled and dissasembled the setup multiple times in the last month, to get the control electronics from the machine into my living room. 
I will take care in the future. I will also note that part number if I ever would need this pin in the future.