One of the issues with non isolation is where is 0 volts. Sometimes I like to use extremes to illustrate a point. Fundamentally, wire has resistance and ohms law applies.
DON"T DO WHAT I DEPICT HERE STRICTLY HYPOTHETICAL TO EXPLAIN
Consider you have one wire from the motor power supply negative to drive X, from drive X to drive Y, from drive Y to drive Z then to the PC. Each drive handle 6A worth of motor. Each drive wire segment is .2 ohms. Ohms law is E=I*R so what you have in drive currents is 6A from the Z segment to the Y segment, 12A in the Y segment to X segment and 18A in the X segment to the Power supply. That means that the 0V at the power supply negative terminal, is actually 3.6V at the first drives negative terminal, 6V at the second drives negative terminal, and 7.2V at the Z drives negative terminal, which is wired to the PC PPort common. In all probablility the PC common is tied to safety gnd, and in good practive the power supply negative terminal is wired to safety gnd (I've never seen one that hasn't and I suspect you can't and be UL approved). So now now have a loop via you ac wiring safety gnd. The wire segment from the Z drive to the pc is probably going to pull a hefty current as there maybe parallel wiring in the cable and connections to/from the db25. You think this is just signal wiring, NBD and the next thing you know you power up, smell smoke, see you parallel cable wire smoking or fused open cause it was too small to handle the current.
If the drives are optically isolated and treated properly that last wire segment (Dark green)doesn't exist.
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