The following figure is a schematic of the Parallel Cable III. It can be used for reference in case you want to build your own download cable.
You must use the recommended lengths for parallel cables. Xilinx cables are typically six feet (approximately two meters) in length between the connector and active circuitry. Keep the wires between the headers and target system as short as possible.
Figure 3.10 Parallel Cable III Schematic |  |