The orientation of the signals inside the cable is very important. There should be three layers to the round cable. The control lines should be in the middle layer, and the TERM power and data lines should be in the outer layer. The distance between each wire must be greater or equal to 0.050 inches. The impedance should be 105 ohms in free air.
Impedance is very important.
This is the signal resistance a wire has under fluctuating currents. Since a round cable has an impedance of 75 ohms and a flat cable around 105 ohms, mixing these two can cause problems. In fact, impedance mismatches are one of the most common causes of SCSI bus problems. Signal reflections tend to occur at sections of the bus where the impedance changes suddenly. As you can see, the tolerances and specifications for the SCSI cables are tight.